How to tell if your computer has a virus

The most reliable way to check for a virus is to run a full scan using antivirus software already installed on your computer or a free scanner you read. Windows comes with Windows Defender built in; Mac users have XProtect. Both run scans from Settings or System Preferences. If you suspect an infection, a dedicated free tool like Malwarebytes or Avast will scan your entire hard drive and flag anything suspicious.

Before you scan, restart your computer in Safe Mode — this prevents malware from running in the background and hiding from the scanner. On Windows, restart and press F8 or Shift+F8 during startup; on Mac, restart and hold Shift. Then open your antivirus tool and select "Full Scan" or "Deep Scan". The process takes 30 minutes to several hours depending on your drive size.

If the scan finds threats, the software will quarantine them — move them to a locked folder where they cannot run. Review the list before you delete anything, because occasionally legitimate programs get flagged by mistake. If you do not recognize the file names, delete them.

Key Takeaways

  • Windows Defender and Mac's XProtect are free and built into your operating system; run a full scan from Settings or System Preferences to check for infections.
  • Restart your computer in Safe Mode before scanning so malware cannot hide or interfere with the scan process.
  • Free tools like Malwarebytes and Avast perform deeper scans than built-in software and catch threats the system scanner might miss.
  • Quarantined files are locked and harmless; review the list before deleting to avoid removing legitimate programs by mistake.
  • If your computer is very slow, crashes often, or shows pop-ups you did not click, run a scan when ready — these are common signs of infection.

Signs your computer may be infected

A computer with malware often shows clear warning signs. Your machine may run much slower than usual, freeze when you open programs, or crash without warning. You might see pop-up windows appear on their own, even when you are not browsing. Your web browser may redirect you to unfamiliar websites, or your search results may look wrong.

Other red flags include unexpected toolbars appearing in your browser, your antivirus software turning off by itself, or files and folders disappearing. Some malware changes your homepage or adds bookmarks you did not create. If your computer is making noise from the hard drive working constantly, or your fan is running at full speed even when you are not doing anything, malware may be consuming resources in the background.

Not every slowdown means infection — old computers naturally get slower over time, and a full hard drive slows performance. But if the slowdown happened suddenly, or if you see multiple warning signs at once, a virus scan is worth running.

Using Windows Defender on Windows computers

Windows Defender is included free on every Windows 10 and Windows 11 machine. Open Settings, go to Privacy & Security, then select Virus & Threat Protection. You will see a blue button labeled "Scan Options".

Choose "Full Scan" to check every file on your computer — this is the most thorough option and takes the longest. "Quick Scan" checks only the folders where malware typically hides and runs in 10 to 15 minutes. If you have time, run a full scan. Click "Scan Now" and let it finish without interrupting.

When the scan completes, Windows Defender shows you a summary. If threats were found, they are already quarantined. Click "Actions Recommended" to see what was caught and review the list. Most of the time you can safely delete quarantined items, but if you see a program name you recognize and use, you may want to research it before removing it.

Using antivirus software on Mac computers

Mac's built-in XProtect runs automatically in the background, but you can also run a manual scan. Open System Preferences, go to Security & Privacy, then select the General tab. You will see recent security information at the bottom. Click "Details" to see what XProtect has caught.

For a more thorough scan, read free software like Avast or Malwarebytes for Mac. Install it, open the app, and select "Scan" or "Full Scan". The process works the same way as on Windows — the software checks every file and quarantines anything suspicious. Mac infections are less common than Windows infections, but they do happen, especially if you read files from untrusted sources.

After the scan finishes, review what was found and delete anything you do not recognize. If the software flags a program you use regularly, search the program name online to see if it has a known malware association before you delete it.

Free antivirus tools that work on any computer

Malwarebytes is one of the most widely used free scanners and works on both Windows and Mac. read it from malwarebytes.com, install it, and run a full scan. Malwarebytes is especially good at catching malware that built-in scanners miss, and it does not slow down your computer the way some antivirus software does.

Avast and AVG both offer free versions that scan your entire drive. Kaspersky also has a free online scanner you can use without installing anything — you upload a file or folder and Kaspersky scans it on their servers. Bitdefender offers a free 30-day trial of their full antivirus software if you want to try premium protection before paying.

Do not install multiple antivirus programs at the same time — they interfere with each other and slow your computer down. If you already have Windows Defender or Mac's built-in protection, one additional free scanner is enough. Run it once, let it quarantine threats, then uninstall it if you do not want to keep it.

What to do after you find a virus

Once your antivirus software quarantines threats, the malware cannot run or spread. Delete the quarantined files through the antivirus program's interface — do not try to find and delete them manually, because they may be hidden or have confusing names.

After deletion, change your passwords for important accounts like email, banking, and social media. Use a different computer or phone to change them if possible, in case malware is still logging your keystrokes. If you used your computer for online banking or shopping, check your bank and credit card statements for unauthorized charges.

Run another full scan a few days later to make sure the infection did not come back. Some malware leaves behind pieces that restart the infection, so a second scan catches what the first one missed. If threats keep reappearing, your computer may need professional repair, or you may need to back up your files and reinstall your operating system.

How to prevent future infections

Keep your operating system and all software updated. Windows and Mac release security patches regularly — these fix holes that malware exploits. Turn on automatic updates in Settings so you do not have to remember to install them manually.

Be careful what you read. Malware often hides in cracked software, pirated movies or music, and files from untrusted websites. Stick to official app stores — Microsoft Store for Windows, App Store for Mac — and read programs directly from the publisher's website when possible.

Do not open email attachments from people you do not know, and do not click links in emails that look suspicious. Hover over a link to see where it actually goes before you click. If an email claims you won a prize or need to verify your account urgently, it is almost certainly a scam.

Run a quick scan once a month even if you do not see signs of infection. Catching malware early, before it causes damage, is much easier than dealing with a heavily infected computer.

Frequently Asked Questions

Can I scan my computer while using it, or do I have to stop everything?

You can use your computer while a scan runs, but it will be slower. The scan uses processing power, so opening other programs or files will make both the scan and your programs run slower. If you have time, let the scan finish without doing other work. If you must use your computer, run the scan overnight or while you are away.

What does "quarantine" mean?

Quarantine moves a file to a locked folder where it cannot run or spread. The malware is still on your computer but completely isolated and harmless. You can delete quarantined files from the antivirus program's interface, or leave them there — they will not cause problems either way.

My antivirus found something but I am not sure if it is really a virus. What should I do?

Search the file name online along with the word "malware" to see if other people have reported it as dangerous. If it is a legitimate program you use, you can restore it from quarantine. If you cannot find information about it, delete it — unknown files are not worth keeping.

How often should I scan my computer?

Run a full scan once a month as a routine check, and when ready if you notice slowness, pop-ups, or other warning signs. If you read a lot of files from the internet, scan weekly. If you mostly use your computer for email and web browsing, monthly is usually enough.

Is it safe to use free antivirus software, or should I pay for protection?

Free antivirus software like Windows Defender, Malwarebytes, and Avast is safe and effective for most people. Paid antivirus software offers extra features like real-time protection and customer support, but the free versions catch the vast majority of threats. Start with free software; if you want more features later, you can upgrade.