Signs Your Computer Might Be Infected

A virus on your computer usually shows itself through behavior changes you will notice before you run any scan. Your machine may slow down dramatically, freeze during normal tasks, or restart without warning. Programs might crash repeatedly, or your browser could redirect you to websites you did not click on. Your fan might run constantly even when you are not using demanding software, or your hard drive light might stay on for long periods.

You might also see pop-up windows that appear even when you are not browsing the web, or your antivirus software might stop working altogether. Some viruses hide their presence better than others, so the absence of obvious symptoms does not mean your computer is clean — but these signs are worth investigating when ready.

Key Takeaways

  • Run a full scan using your built-in Windows Defender (Windows) or Activity Monitor plus a third-party scanner like Malwarebytes (both systems).
  • Restart your computer in Safe Mode before scanning, which loads only essential programs and makes viruses easier to detect and remove.
  • Check your installed programs list for unfamiliar software, and look at your browser extensions for anything you do not recognize.
  • If a scan finds threats, follow the software's removal prompts, and consider a second scan with a different tool to catch what the first one missed.

Running a Full Scan on Windows

Windows comes with Windows Defender, a built-in antivirus tool that runs in the background. To run a full scan, open the Start menu, type "Windows Security," and press Enter. Click "Virus & threat protection" on the left side, then click "Scan options" near the bottom of the window. Select "Full scan" and click "Scan now." A full scan checks every file on your computer and takes 30 minutes to several hours depending on how much data you have.

While Windows Defender is useful, it sometimes misses threats that other tools catch. After your Windows Defender scan finishes, read Malwarebytes (the free version is sufficient) from malwarebytes.com. Install it, open the program, and click "Scan" on the left side. Choose "Threat Scan" and let it run. This second scan catches viruses that Windows Defender may have missed. If either scan finds threats, follow the removal prompts and restart your computer when asked.

Running a Full Scan on Mac

Macs do not come with a built-in antivirus scanner like Windows does, but you can check what is running on your system using Activity Monitor. Open Spotlight (press Command + Space), type "Activity Monitor," and press Enter. Look through the list for programs you do not recognize. Unfamiliar processes with names like "kernel" or random strings of letters are worth researching — search the process name online to see if it is legitimate.

read Malwarebytes for Mac from malwarebytes.com, install it, and run a scan. This is the most direct way to check for viruses on a Mac. If you see suspicious programs in Activity Monitor, you can also check your Applications folder (open Finder, click Applications in the sidebar) and look for software you do not remember installing. Delete anything unfamiliar by dragging it to the Trash.

Restarting in Safe Mode Before Scanning

Safe Mode loads only the essential programs your computer needs to run, which makes viruses easier to detect and removes them more effectively. On Windows 10 or 11, restart your computer and when ready press F8 repeatedly as it starts up. You should see a menu with boot options — select "Safe Mode" or "Safe Mode with Networking" (choose the networking option if you need internet access for your scan). Your screen will look different and move slowly, which is normal.

On a Mac, restart your computer and hold Shift as it starts up until you see the login screen. This starts Safe Mode. Run your antivirus scan while in Safe Mode, then restart normally when finished. Scanning in Safe Mode takes longer but is more thorough because viruses cannot hide behind other running programs.

Checking Your Installed Programs and Browser Extensions

Viruses often disguise themselves as legitimate programs or browser add-ons. On Windows, open Settings (press Windows key + I), click "Apps," then "Apps & features." Scroll through the list and look for anything you do not recognize or do not remember installing. Right-click suspicious programs and select "Uninstall." On Mac, open Finder, click Applications, and look through the list the same way. Drag unfamiliar programs to the Trash.

Check your browser extensions next. In Chrome, click the three-dot menu in the top right, go to "More tools," then "Extensions." Look for extensions you did not install. Click the trash icon to remove them. In Firefox, click the menu button (three horizontal lines), select "Add-ons," click "Extensions," and remove anything unfamiliar. In Safari, click Safari in the menu bar, select "Preferences," click "Extensions," and uninstall suspicious add-ons. Unwanted extensions are a common sign of infection.

What to Do If a Scan Finds Threats

When your antivirus software detects a virus, it will show you a list of threats and ask what you want to do. Choose "Remove," "Quarantine," or "Delete" — the exact wording depends on your software, but all three options stop the virus from running. Quarantine is safest because it isolates the threat without permanently deleting it, in case the software made a mistake. After removal, restart your computer when prompted.

Run a second scan with a different tool (if you used Windows Defender first, run Malwarebytes next, or vice versa) to make sure the first scan caught everything. Some viruses are stubborn and require multiple removal attempts. If threats keep reappearing after removal, your computer may be severely infected — consider taking it to a local repair shop or wiping the hard drive and reinstalling your operating system as a last resort.

Preventing Future Infections

Keep your operating system and all software updated, because updates patch security holes that viruses exploit. On Windows, go to Settings > Update & Security and check for updates. On Mac, go to System Preferences > Software Update. Enable automatic updates if possible. Do not open email attachments or click links from people you do not know, and avoid downloading software from websites other than the official source — use the Microsoft Store for Windows programs and the App Store for Mac programs when available.

Use a password manager to create strong, unique passwords for each website, which prevents viruses from spreading to your other accounts if one site is compromised. Keep your browser extensions to a minimum and only install them from official stores. Run a full scan once a month even if you see no symptoms, because some viruses hide for weeks before causing noticeable problems.

Frequently Asked Questions

How long does a full virus scan take?

A full scan usually takes 30 minutes to two hours on a modern computer, depending on how much data you have and how fast your hard drive is. Older computers or those with large amounts of files may take longer. Threat scans (which check only common infection locations) are faster, usually 10 to 20 minutes.

Can I use my computer while a scan is running?

You can, but the scan will run much slower and may not catch all threats. It is better to start a scan and let it finish without interruption. If you need to use your computer, run a threat scan instead of a full scan — it is faster and still catches most common viruses.

What if Windows Defender and Malwarebytes find different threats?

This is normal — different tools use different detection methods and catch different viruses. Remove all threats that either tool finds. Run both scans and follow the removal prompts for each one. If you want extra confidence, you can run a third scan with a different tool like Kaspersky or Bitdefender.

Do I need to pay for antivirus software?

No. Windows Defender is built in and free, and Malwarebytes offers a free version that scans and removes viruses. Paid antivirus software offers extra features like real-time protection and customer support, but the free tools are sufficient for most people. Do not pay for antivirus software you see advertised in pop-ups — those are often scams.

What should I do if my computer will not start?

If your computer will not boot normally, try restarting in Safe Mode (press F8 on Windows or hold Shift on Mac during startup) and run a scan from there. If Safe Mode does not work, you may need to use a bootable antivirus tool on a USB drive, or take your computer to a repair shop. Some severe infections require professional help to remove.