Start with what's already on your computer

Windows comes with a built-in antivirus tool called Windows Defender (or Microsoft Defender on newer versions). This runs automatically in the background, but you can run a manual scan whenever you want. If you're using Windows, you likely already have protection running — you just need to know how to check it and run a deeper scan if you're worried about an infection.

Before you download anything new or pay for a third-party scanner, check what you have. Many people buy antivirus software they don't need because they didn't realize their system already had one.

Key Takeaways

  • Windows Defender is built into Windows and runs automatically, so check it first before buying antivirus software.
  • A quick scan takes 10 to 15 minutes and checks the most common infection spots; a full scan takes an hour or more but checks every file.
  • Run your scan when you're not using the computer for other tasks, because scanning slows everything down.
  • If Windows Defender finds something, it quarantines it automatically — you don't have to remove it yourself.
  • Third-party scanners like Malwarebytes can catch things Defender misses, but you only need them if Defender finds nothing and you still suspect a problem.

How to open Windows Defender and check its status

Click the Windows Start button (bottom left corner) and type "Windows Security" into the search box. Open the Windows Security app. You'll see a home screen with several options. Look for Virus & threat protection — click it.

On this screen you can see whether real-time protection is turned on (it should be green and say "On"). You can also see when the last scan ran and what the current threat status is. If it says "No threats found" and the date is recent, your computer has been checked.

Running a quick scan versus a full scan

In the Virus & threat protection window, look for Scan options. Click it. You'll see four choices: Quick scan, Full scan, Custom scan, and Offline scan. For most situations, start with Quick scan. This checks your Downloads folder, temporary files, and other places where viruses commonly hide. A quick scan usually takes 10 to 15 minutes.

If the quick scan finds nothing but you're still concerned, run a Full scan. This checks every file on your computer and can take 45 minutes to two hours depending on how much storage you have. You don't need to do this often — once a month or when you suspect a real problem is enough. Run it when you're not using your computer for other work, because scanning uses a lot of processing power and will slow everything down.

A Custom scan lets you pick specific folders to check — useful if you think a problem is in one location. An Offline scan restarts your computer and scans before Windows fully loads, which can catch stubborn infections, but most people never need this.

What to do if a threat is found

If Windows Defender finds something, it usually quarantines it automatically — meaning it moves the file to a safe place where it can't run. You'll see a notification telling you what was found. In most cases, you don't have to do anything else. The threat is contained.

If you want to see what was quarantined, go back to Virus & threat protection and look for Threat history. Click it. You'll see a list of everything Defender has caught. Most of the time these are false alarms — files that looked suspicious but weren't actually dangerous. You can leave them quarantined or permanently remove them. Leaving them quarantined is safer if you're not sure.

When to use a second scanner

If Windows Defender runs a full scan and finds nothing, but your computer is still acting strange — running slowly, showing unexpected ads, or crashing — you can run a second opinion scan with Malwarebytes. This is a separate tool that sometimes catches things Defender misses. Download it from malwarebytes.com, install it, and run a scan. The free version works for one-time scans.

Do not run two antivirus programs at the same time on your computer. They will conflict with each other and slow your system down. Use Defender as your main protection, and only run Malwarebytes or another scanner occasionally if you suspect a problem.

Signs your computer might have a virus

Before you panic and run multiple scans, know what actually suggests an infection. Real warning signs include: your computer running much slower than normal, programs opening on their own, your browser homepage changing without you changing it, constant pop-up ads appearing even when you're not browsing, or your antivirus being turned off by itself.

Not every slow computer has a virus. A full hard drive, too many browser tabs, or old hardware can all cause slowness. But if slowness started suddenly and your antivirus is off, run a full scan. If your browser is hijacked (homepage or search engine changed), that's worth a scan too. Strange noises from your hard drive or a completely frozen screen are usually hardware problems, not viruses.

Keeping your computer protected between scans

Scanning is important, but prevention is easier than cleanup. Keep Windows Defender turned on at all times — check the Virus & threat protection screen once a month to make sure real-time protection still shows "On". Install Windows updates when they appear; many updates patch security holes that viruses use. Don't open email attachments from people you don't know, and don't download files from websites that look suspicious.

If you use a web browser, keep it updated too. Chrome, Firefox, and Edge all auto-update, but older versions have known security problems. You can check your browser version in its settings menu under "About" — if an update is available, it will install automatically when you close and reopen the browser.

Frequently Asked Questions

How often should I scan my computer for viruses?

If Windows Defender is on and you're careful about what you download, a quick scan once a month is enough. If you download files from untrusted sources or visit risky websites regularly, scan weekly. If you suspect an active infection, scan immediately.

Will a virus scan remove viruses automatically?

Windows Defender quarantines threats automatically, which stops them from running. You don't have to manually delete anything. If you want to permanently remove a quarantined file, you can, but leaving it quarantined is safe and takes no action from you.

Can I use my computer while it's scanning?

Technically yes, but don't. Scanning uses a lot of your computer's processing power, so using it at the same time will make both the scan and your work slower. Start a scan when you're about to take a break or before you leave your desk.

What if Windows Defender is turned off?

Go to Virus & threat protection in Windows Security and look for "Manage settings." If real-time protection shows "Off," click it to turn it back on. If it won't stay on, a virus may have disabled it — restart your computer in Safe Mode and try again, or run an offline scan.

Do I need to buy antivirus software if I have Windows Defender?

No. Windows Defender is strong enough for most people. You only need paid antivirus if you work with sensitive data, run a business, or have had repeated infections that Defender didn't catch. For everyday use, Defender plus safe browsing habits is sufficient.