Signs your computer might be infected

A virus is malicious software designed to copy itself and spread to other files or devices. The most common signs are: your computer runs much slower than usual, programs crash or freeze without warning, your browser homepage or search engine changes on its own, you see pop-up windows you did not click on, your antivirus software stops working or you cannot open it, or your hard drive makes constant noise as if it is working overtime.

You might also notice unfamiliar programs in your Start menu or taskbar, your mouse cursor moves on its own, your keyboard types characters you did not press, or your internet connection drops repeatedly. Some viruses hide their presence entirely, so the absence of obvious symptoms does not mean your computer is clean.

Key Takeaways

  • Slow performance, unexpected pop-ups, and programs that crash are the most common signs of a virus, though some infections cause no visible symptoms.
  • Run a full scan with your existing antivirus software first—most Windows computers come with Windows Defender built in, and most Macs come with XProtect.
  • If your antivirus software will not open or run, restart your computer in Safe Mode (Windows) or Recovery Mode (Mac) before scanning.
  • Malware removal tools like Malwarebytes can find threats that standard antivirus software misses, and many offer a free scan.
  • If a scan finds nothing but your computer still behaves oddly, the problem may be hardware failure, a full hard drive, or too many programs running at startup rather than a virus.

How to run a virus scan on Windows

Windows 10 and Windows 11 come with Windows Defender already installed. Open the Start menu, type "Windows Security" and press Enter. Click "Virus & threat protection" on the left side. Under "Current threats," click "Scan options," then select "Full scan" and click "Scan now." A full scan takes 30 minutes to several hours depending on how much data is on your drive.

If Windows Defender is already running a scan, you will see a progress bar. Do not shut down your computer during the scan. When it finishes, Windows will show you a list of any threats found. If threats are detected, Windows will ask whether to remove them—click "Remove" or "Quarantine" to let Windows handle them.

If you cannot open Windows Security or Windows Defender, restart your computer in Safe Mode. Press the power button to shut down completely. Turn the computer back on and when ready press F8 repeatedly (on some newer computers, hold Shift while clicking Restart from the login screen, then choose "Troubleshoot" and "Advanced options" and "Startup Settings"). Select "Safe Mode with Networking" and try opening Windows Security again.

How to run a virus scan on Mac

Macs come with XProtect, which scans files automatically in the background. To run a manual scan, open System Preferences (or System Settings on newer Macs), click "Security & Privacy," then the "General" tab. You will see a message about the last time XProtect ran. Unfortunately, macOS does not offer a way to manually trigger a full system scan through the Settings app.

Instead, read and run Malwarebytes for Mac (malwarebytes.com), which offers a free scan. Open the downloaded file, drag Malwarebytes into your Applications folder, then open Applications and double-click Malwarebytes. Click "Scan" and let it run. If threats are found, Malwarebytes will show you what they are and ask whether to remove them.

Using additional malware removal tools

Malwarebytes is a dedicated malware scanner that often finds threats standard antivirus software misses. read it from malwarebytes.com, install it, and run a free scan. The free version scans and shows you what it finds; the paid version removes threats automatically. Many people run Malwarebytes alongside their regular antivirus software because the two tools use different detection methods.

Other widely used free scanners include HitmanPro (hitmanpro.com) and Kaspersky Rescue Disk (kaspersky.com), though Kaspersky requires burning the tool to a USB drive and booting from it—only do this if you are comfortable with that process. If you use one of these tools and it finds threats, follow its instructions to remove them. You do not need to run all three; one thorough scan with Malwarebytes is usually enough.

What to do if a scan finds a virus

When antivirus software detects a threat, it will ask what you want to do. The safest option is always "Remove," "Quarantine," or "Delete"—these options isolate or destroy the infected file so it cannot spread. Do not click "Ignore" or "Allow" unless you are absolutely certain the file is safe, which is rare.

After removal, restart your computer. Then run another scan to confirm the threat is gone. If the same threat appears again after removal, the virus may have hidden copies in multiple locations, or your computer may be reinfected from the internet. In that case, consider disconnecting from the internet, running the scan again, and then checking your browser extensions and startup programs (see the next section).

Check your browser and startup programs

Some viruses and malware work by installing browser extensions or hijacking your search engine. Open your web browser (Chrome, Firefox, Edge, or Safari), look for the menu icon (usually three dots or lines), and find "Extensions" or "Add-ons." Remove anything you do not recognize or did not intentionally install. Then check your homepage and search engine settings—if they have changed to something unfamiliar, change them back to your preferred search engine.

On Windows, viruses sometimes add themselves to your startup programs so they run every time you turn on your computer. Open the Start menu, type "Task Manager," and press Enter. Click the "Startup" tab. Look through the list for programs you do not recognize. Right-click any suspicious entry and select "Disable." On Mac, go to System Preferences (or System Settings), click "General," then "Login Items," and remove anything unfamiliar from the list.

When to call a professional

If you have run a full scan with Windows Defender or Malwarebytes and found nothing, but your computer still runs slowly or behaves strangely, the problem may not be a virus. A full hard drive, too many programs running at startup, outdated drivers, or failing hardware can all cause the same symptoms. A local computer repair shop can diagnose the real cause in 30 minutes to an hour.

Also call a professional if: your antivirus software will not open even in Safe Mode, you see a message saying your computer is locked or infected and demanding payment, or you cannot remove a threat after multiple scans. These situations often require specialized tools or manual removal that go beyond what a standard user can do safely.

Frequently Asked Questions

Can a virus spread from my computer to my phone or tablet?

Most computer viruses cannot directly infect phones or tablets because they run different operating systems. However, if a virus steals your passwords, someone could use them to access your phone accounts. Keep your phone and tablet updated with the latest security patches, use a strong unique password for each account, and consider using a password manager.

Is it safe to use my computer while a virus scan is running?

You can use your computer during a scan, but it will run much slower because the scan uses processing power. Avoid opening large files or running demanding programs. The scan will take longer if you are actively using the computer, so it is usually faster to start the scan and then step away.

What if I think I have a virus but I am afraid to run a scan?

Running a scan is safe—antivirus software is designed to detect and remove threats without harming your files. The worst that can happen is the scan finds nothing, which means your computer is clean. If you are still worried, take your computer to a local repair shop and let them run the scan for you.

Do I need to pay for antivirus software?

No. Windows Defender (built into Windows) and XProtect (built into Mac) are free and sufficient for most users. Malwarebytes offers a free scan. Paid antivirus software adds features like real-time monitoring and automatic removal, but the free tools catch most common threats.

How often should I scan my computer for viruses?

If your antivirus software is running in the background (which Windows Defender and XProtect do by default), you do not need to manually scan often. Run a full scan once a month or whenever you notice your computer behaving oddly. If you read files from untrusted sources frequently, scan weekly.