How to know if your PC or Mac has a virus

Over the years, viruses have become more complex and can compromise your system in new ways, leading to performance issues and data breaches. If your computer is running slowly, there are apps you don’t recognize, or you see pop-ups, it could mean it’s been infected. 

In this guide, we will explain how to know if your computer has a virus, how to confirm its presence, what to do about it, and how to protect yourself in the future.

Jump to…
12 signs that your computer is infected with a virus
How does a computer get a virus?
Types of computer viruses
The impact of a computer infection
How to check for and get rid of computer viruses
How to avoid computer viruses and malware

12 signs that your computer is infected with a virus

Computer acting up? Here are some signs that your device’s problem is related to a virus.

1. Slow-running computer

A sudden drop in your computer’s performance is often the first sign of a virus infection. If once instantaneous tasks are now taking longer, or if your system struggles with basic operations, malicious software may be consuming your computer’s resources.

What it looks like: You might notice applications taking longer to launch or files taking longer to open. Even simple actions could become frustratingly slow, like opening a new browser tab.

2. Screen pop-ups

Frequent pop-ups appearing on your screen are a classic symptom of adware, a type of malware. These pop-ups can be annoying and often persist despite your attempts to close them. They may advertise products or prompt you to click on links that could lead to further malware infections if interacted with.

What it looks like: These pop-ups can range from advertisements to false warnings about your computer’s security. 

Get ExpressVPN

3. Disappearing files

If files start to disappear or move without your intervention, it’s a clear sign that something is amiss. This suggests that a virus is manipulating your files, either by hiding them, deleting them, or moving them to unfamiliar locations as part of its activities. Viruses can delete or encrypt your files, making them inaccessible. This behavior can be particularly damaging, as it may result in the loss of important personal or work-related data.

What it looks like: You may notice that documents, photos, or other files are no longer where you left them, or they might be missing entirely. 

4. Antivirus software not working

If your antivirus software is disabled or not functioning correctly, it could be the result of a virus or malware attack. Malicious programs often target security software to avoid detection and removal, leaving your computer vulnerable to further infections.

What it looks like: You might find that your antivirus program refuses to start, or it may shut down unexpectedly after launching. Updates and scans could fail to run, or the software might display error messages. 

5. Suspicious programs

Discovering programs on your computer you don’t recall installing is a red flag indicating a potential virus infection. These programs may be malware disguised as legitimate software installed without your knowledge to carry out harmful activities or to breach your privacy. They can consume system resources, monitor your activities, or open backdoors for other malware to enter.

What it looks like: You may notice unfamiliar applications running on startup or see new icons on your desktop or in your system tray. 

6. System blocking

If you find yourself suddenly locked out of your computer or specific functions are no longer accessible, it strongly indicates a ransomware attack. Ransomware is a type of malware that restricts access to your system or personal files until a ransom is paid. This form of malware is particularly malicious, as it directly threatens your personal or business data and leverages it for financial gain.

What it looks like: You may encounter a message on your screen demanding payment to restore access to your computer or files. 

7. Battery draining

Rapid battery depletion on your laptop can be a symptom of malware running in the background. Malicious software often uses significant system resources, leading to abnormal battery drain even when your computer is not performing resource-intensive tasks. These processes might be malware attempting to communicate with external servers, mine cryptocurrencies, or perform other unauthorized activities that consume more power than regular operations.

What it looks like: If your battery life has suddenly decreased significantly without a change in usage patterns, it could indicate that hidden processes are active. 

8. Home page switching

If your web browser’s default home page changes without your input, it’s a sign that a browser hijacker may have infected your computer. These types of malware alter your browser settings to redirect your searches and homepage to specific websites, often for advertising revenue or to spread further malware.

What it looks like: You might open your browser to find that your usual home page is replaced with a different one, or your searches are redirected through unfamiliar search engines. 

9. Frequent system crashes

Experiencing frequent system crashes or the “blue screen of death” on PCs can indicate that your computer is infected with a virus. Malware can cause instability in your system, leading to crashes during normal operations or when attempting to run specific programs.

What it looks like: If your computer suddenly begins to crash more often, especially if these crashes occur without a clear pattern or cause, it’s a sign that malicious software might be interfering with your system’s normal functions.

10. Problems unexpectedly shutting down or restarting

If your computer begins to shut down or restart without warning, it could be a sign of a virus or malware infection. These unexpected actions can disrupt your work and data and could indicate that malicious software is interfering with your system’s operations. This behavior can be particularly concerning if it happens frequently or interrupts your usage unless it’s a system update. It suggests that a virus may be attempting to evade detection or complete its installation by rebooting your system.

What it looks like: You might find your computer turning off or rebooting at odd times, even when you don’t want it to, and at random. 

11. Your mouse, keyboard, or devices appear to act on their own

Observing your mouse cursor moving independently, keystrokes registering without your input, or external devices activating on their own can be alarming. These phenomena often indicate that malware has compromised your computer, potentially giving remote access to an unauthorized user.

What to know: If you notice unexpected actions like your mouse moving on its own, random text appearing as if typed by an unseen hand, or devices connecting and disconnecting without your intervention, it’s a strong sign of a security breach.

12. Unexpected apps are running in the background

Applications running in the background that you did not open can be a sign of a malware infection. These applications can consume system resources, slow down your computer, and may be part of a malicious operation such as data mining, logging your keystrokes, or spying on your activities.

What to know: You might notice unfamiliar processes in your task manager or unexpected software consuming CPU and memory resources. This situation often means that malware has been installed on your system without your knowledge, performing tasks that compromise your computer’s performance and your privacy. 

How does a computer get a virus?

Understanding how a computer becomes infected is crucial for preventing attacks and maintaining your system’s health. Viruses can infiltrate your computer in various ways, often exploiting user actions and vulnerabilities within the system or software.

• Email Attachments: One of the oldest methods for spreading viruses, malicious email attachments can infect your system when opened or downloaded.
• Downloaded Software: Downloading and installing software from untrusted sources can introduce viruses. This includes both applications and multimedia content.
• Removable Media: USB drives and other removable media can contain viruses that automatically execute when connected to your computer.
• Phishing Scams: Clicking on links in phishing emails or messages can direct you to sites that download malware to your system.
• Exploiting Software Vulnerabilities: Hackers can use software vulnerabilities to inject malware into your computer. Keeping software up to date is crucial to prevent this.
• Network Attacks: Viruses can spread across networks, infecting computers connected to the same network without any user interaction.
• Malicious Websites: While rarer, simply visiting a compromised website can lead to automatic downloads of malware without your knowledge. It could also trick you into installing malware yourself.

Hackers have a lot of ways to target you nowadays. But if you can recognize these methods, you’re already taking a big step in the right direction. Also, you can significantly reduce the risk of virus infection by exercising caution online and following good practices such as regularly updating your software and using reputable antivirus software.

Types of computer viruses

Computer viruses come in various forms designed to disrupt, damage, or gain unauthorized access to your system. Understanding these differences can help you recognize potential threats and implement effective countermeasures. 

Overwriting virus

An overwriting virus is a malicious program designed to infect files and replace their contents with its own code, potentially leading to losing the original data in those files. Unlike other types of malware that might hide within a file to avoid detection, overwriting viruses make their presence known by rendering the infected files unusable.

Network virus

A network virus is a particularly virulent type of malware that spreads across computer networks, exploiting vulnerabilities in network protocols or software to infect multiple systems. 

It can propagate without user intervention, moving swiftly through shared folders, email, and even instant messaging applications. Once inside a network, it seeks to infect as many machines as possible, often leading to widespread system operations and data integrity disruption. 

Browser hijacker virus

A browser hijacker virus is a type of malware designed to modify a user’s browser settings without their permission. This can result in the home page and default search engine being changed to direct traffic to unwanted or malicious websites. 

These hijackers often insert additional advertisements, pop-ups, and toolbars into the browser, which can compromise browsing speed and overall system performance. The primary goal of a browser hijacker is to generate revenue through ad clicks and traffic redirection.

Polymorphic virus

A polymorphic virus is a sophisticated type of malware that can change its code or signature every time it infects a new system, making it particularly difficult for antivirus programs to detect and remove. 

This mutating ability helps the virus evade detection by security software that relies on identifying malware signatures—a digital fingerprint of known malicious files. Polymorphic viruses can infect systems through various means, including email attachments, downloadable software, and compromised websites.

The impact of a computer infection

The impact of a computer infection extends beyond mere inconvenience and poses severe threats to your data integrity, privacy, and the overall performance of your system.

• Loss of Performance: Malware can significantly slow down your computer by using valuable system resources, making everyday tasks frustratingly sluggish.
• Loss of Bandwidth: Certain types of malware, like worms and trojans, use your internet connection to propagate, leading to reduced internet speed and increased latency.
• Loss of Function: Viruses can corrupt or delete critical system files, leading to errors, system instability, and the inability to use certain functionalities.
• Exposure to Other Dangerous Software: An initial infection can be a gateway for additional malware, compounding the risks and potential damage.
• Loss of Information: Malware such as ransomware can encrypt or delete your personal files, leading to significant data loss.
• Breach of Privacy: Spyware and keyloggers can capture sensitive information, exposing you to identity theft and privacy violations.

How to check for and get rid of computer viruses

While viruses can be annoying, there are concrete steps you can take to check for and completely eliminate viruses. We’ll go through the most important ones and bear in mind that the specifics of your device could change, but they all apply to every computer.

1. Run a full system scan

Initiating a full system scan with high-quality antivirus software can identify and eliminate most malware. If you leave it on, you can also protect yourself from future infections and detect hidden viruses and malware.

2. Restore to an earlier backup

Restoring your computer to a pre-infection backup can effectively reverse damage caused by a virus infection. You must do a full system backup and not just of your files, as malware usually hides inside system folders. File backups can only help you restore your files, not system integrity.

3. Delete temporary files

Deleting temporary files clears out potential hiding spots for simple malware and frees up disk space, improving subsequent virus scans’ efficiency. While you won’t remove any complex threats, it’s still a good idea to do this. Ensure that you clear your browser cache and your system temporary files.

4. Go Safe Mode

Booting your computer in Safe Mode restricts the system to running only the essential programs and services. This limited startup environment can prevent viruses and malware from activating, making it easier to diagnose and remove infections without interference.

To boot on safe mode on Windows, follow these steps:

1. Press the Windows logo key + R.
2. Type msconfig in the Open box and then select OK.
3. Select the Boot tab.
4. Under Boot options, select the Safe boot checkbox.

Booting into safe mode for Apple computers will depend on whether you have an Intel or Apple Silicon device. For Intel Macs, follow these steps:

1. Power up or restart your Mac.
2. As soon as it starts, press and hold Shift.
3. Release Shift when you see the login window.

For Apple Silicon Macs, follow these steps:

1. Turn off your Mac.
2. Press and hold the power button until you see the startup options.
3. Choose your system disk.
4. Press the Shift key, and click Continue in Safe Mode.

5. Reinstall your operating system

Reinstalling your operating system is a definitive measure to eliminate viruses or malware that are deeply embedded in your system. This process involves wiping your hard drive clean and installing a fresh version of the OS, which ensures the removal of all malicious software.

6. Download and install legitimate antivirus software

Downloading and installing legitimate antivirus software is among the best measures to protect your computer against viruses and malware. Choose a reputable antivirus program that offers comprehensive protection, including real-time scanning, threat detection, and removal capabilities.

7. Disconnect from the internet

This action prevents the spread of the virus to other devices on your network and stops the malware from communicating with any external controllers or downloading additional malicious payloads. While it will not remove the infection, cutting off internet access can significantly reduce the potential impact of the virus and provide a safer environment for cleaning and repairing your infected system.

8. Change all your passwords after virus removal

Changing all your passwords won’t help with removing the virus, but it’s an essential step, especially if the malware has the potential to capture keystrokes or access personal information. Update your passwords from a different, secure device to prevent immediate recapture by any virus remnants.

How to avoid computer viruses and malware

Preventing computer viruses and malware requires proactive measures and an understanding of common threats. These are the most important measures you can take:

Regularly update your apps and operating systems

Keeping your applications and operating system up to date is important for security because developers regularly release updates that patch vulnerabilities, which, if left unpatched, can be exploited by viruses and malware. Ensuring your software is always the latest version significantly reduces the risk of infections and enhances your system’s overall security.

Read more: Auto app updates: Pros and cons, and how to turn them on

Use a pop-up blocker

Activating a pop-up blocker in your web browser is a simple but effective way to reduce the risk of accidentally clicking on malicious advertisements or links that could lead to virus infections. Most modern browsers include built-in pop-up blockers that can be easily enabled in the settings.

Learn the warning signs of a phishing scam

Some infections often originate from deceptive emails or messages. Phishing attempts typically mimic legitimate communications but contain malicious links or requests for personal information. Being suspicious about unsolicited emails, checking sender addresses, and avoiding clicking on suspicious links are essential to protect yourself from these scams.

Read more: How to spot common red flags in phishing emails

Only download apps and files from official sources

Trusted websites and official app stores rigorously check files and applications developers submit to them to prevent malware from being distributed through their platforms. This is something you don’t get if you download directly from a website, so by sticking to these sources, you significantly lower the risk of inadvertently installing malicious software on your device.

Get ExpressVPN