Signs your computer might have a virus

A virus or malware infection usually shows itself through behavior changes you'll notice before you run any scan. Your computer may run noticeably slower, freeze during normal tasks, or take much longer to start up. Programs may crash without warning, or your browser may redirect you to websites you didn't click on.

You might see pop-up windows appearing constantly, even when you're not browsing. Your antivirus software may stop working or refuse to update. Files could disappear, your hard drive may make unusual grinding sounds, or you might notice unfamiliar programs in your Applications folder (Mac) or Programs list (Windows). Some infections cause your computer to overheat because malware runs constantly in the background.

If you notice any of these signs, you don't need to panic — many are also caused by legitimate software conflicts or hardware problems. But they're worth investigating with a scan.

Key Takeaways

  • Windows Defender (built into Windows) and Malwarebytes are reliable free tools that scan for viruses without requiring payment upfront.
  • Run a full system scan, not a quick scan, because quick scans only check common infection locations and miss many threats.
  • Disconnect from the internet before scanning if your computer is severely infected, because some malware tries to download additional threats during the scan.
  • If a scan finds nothing but your computer still behaves strangely, the problem may be hardware failure, a corrupted Windows installation, or a legitimate program using heavy resources.

How to scan with Windows Defender on Windows

Windows Defender is built into every Windows computer and requires no download. Open the Windows Security app by typing "Windows Security" into the search box at the bottom left of your screen and pressing Enter. Click "Virus & threat protection" on the left side.

Click "Scan options" and select "Full scan," then click "Scan now." A full scan checks your entire hard drive and takes 30 minutes to several hours depending on how much data you have. Do not interrupt it or restart your computer while it runs. When the scan finishes, Windows Defender will report what it found and ask whether you want to remove threats. Click "Remove" or "Quarantine" to let it clean the infections.

If Windows Defender finds nothing but your computer still runs slowly, run the scan again in a few days — some infections hide from the first scan. If it finds threats but your computer doesn't improve after removal, the slowness may come from something else.

How to scan with Malwarebytes on Windows or Mac

Malwarebytes is a free tool that catches some threats Windows Defender misses. Go to malwarebytes.com, click "Free Download," and run the installer. Open Malwarebytes after installation and click "Scan" on the left side. The program will automatically run a full scan.

Malwarebytes scans faster than Windows Defender — usually 15 to 45 minutes — and shows results as it finds them. When the scan finishes, review what it found. Malwarebytes will recommend removing threats; click "Quarantine" to remove them. You can run Malwarebytes on both Windows and Mac computers.

The free version of Malwarebytes does not include real-time protection, meaning it only scans when you tell it to. The paid version watches your computer constantly and blocks threats as they arrive, but the free version is enough to check whether you currently have an infection.

What to do if a scan finds threats

If Windows Defender or Malwarebytes finds malware, let the program remove it. Both tools quarantine infected files, meaning they move them to a safe location where they can't run. You can safely delete quarantined files — the program will not accidentally remove something important.

After removal, restart your computer. Run a second scan a few days later to make sure the infection is gone — some malware leaves behind pieces that regenerate. If the second scan finds nothing, the threat is removed.

If a scan finds threats but your computer still behaves strangely after removal, you may have multiple infections. Run Malwarebytes if you used Windows Defender first, or run Windows Defender if you used Malwarebytes first — different tools catch different threats. If both scans find nothing and your computer is still slow, the problem is likely not a virus.

When to suspect the problem is not a virus

If you've run a full scan with both Windows Defender and Malwarebytes and both found nothing, but your computer is still slow, the issue is probably not malware. Check your hard drive space by opening File Explorer (Windows) or Finder (Mac) and looking at how much storage you have left. If you're using more than 90 percent of your hard drive, your computer will slow down significantly even without a virus.

Open Task Manager on Windows by pressing Ctrl+Shift+Esc, or Activity Monitor on Mac by pressing Command+Space, typing "Activity Monitor," and pressing Enter. Look at the CPU and Memory columns to see which programs are using the most resources. If a program you recognize is using 80 percent or more of your CPU or memory, that program is the problem, not a virus.

Hardware failure also causes slowness and freezing. If your hard drive makes clicking or grinding sounds, or if your computer overheats even when you're not running programs, the hard drive may be failing. Back up your important files immediately and take your computer to a repair shop for diagnosis.

How to prevent future infections

Keep Windows or macOS updated by checking for updates regularly. On Windows, go to Settings > Update & Security > Windows Update and click "Check for updates." On Mac, go to System Preferences > Software Update. Install updates as soon as they're available — they patch security holes that malware exploits.

Do not download programs from unfamiliar websites. Use the Microsoft Store on Windows or the App Store on Mac when possible, because both check programs before listing them. If you download from a website directly, make sure it's the official website for that program — malware often hides on fake download pages that look almost identical to the real ones.

Be cautious with email attachments and links, especially from people you don't know. Do not open attachments unless you expected them, and do not click links in emails asking you to "verify your account" or "confirm your password" — legitimate companies never ask for passwords by email.

Frequently Asked Questions

Is Windows Defender enough, or do I need to buy antivirus software?

Windows Defender is sufficient for most people and catches the majority of common threats. Running Malwarebytes alongside it once a month provides extra protection without cost. Paid antivirus software offers real-time monitoring and faster scans, but the free combination of Windows Defender and occasional Malwarebytes scans handles most infections.

Why does my scan say "threats found" but won't let me remove them?

Some malware locks itself so the antivirus can't remove it while Windows is running. Restart your computer in Safe Mode, which loads only essential Windows files, then run the scan again. On Windows, restart and press F8 repeatedly before the Windows logo appears, then select "Safe Mode." On Mac, restart and hold Shift until you see the login screen.

Can I scan my computer without restarting?

Yes, but a restart scan is more thorough. If you run a scan without restarting and it finds nothing, but your computer still behaves strangely, restart and scan again. Some malware only loads when Windows starts, so a restart scan will catch it.

How often should I scan for viruses?

If you use Windows Defender with real-time protection enabled, it watches your computer constantly and you don't need to manually scan. If you want extra assurance, run Malwarebytes once a month. If you notice signs of infection, scan immediately rather than waiting for your scheduled scan.

What if I think my computer is infected but I'm afraid to turn it on?

Turning on your computer to scan it is safe — the scan runs in a controlled way and won't spread the infection. If you're concerned about data theft, disconnect from the internet before scanning by unplugging your ethernet cable or turning off Wi-Fi. The malware can't send your data anywhere without an internet connection.