The fastest way to check for viruses on your PC

The fastest check is to run Windows Defender, which comes built into Windows 10 and Windows 11 and scans your system without installing anything new. Open the Start menu, type "Windows Security," and click the app. Select "Virus & threat protection," then click "Scan options" and choose "Full scan." This takes 30 minutes to several hours depending on how many files you have, but it checks every file on your drive.

If you want a faster initial check, Windows Defender also offers a "Quick scan" that checks only the places where malware most often hides — your startup programs, temporary files, and system folders. A quick scan usually finishes in 5 to 15 minutes. If it finds nothing, you likely do not have an active infection. If it finds something, run the full scan to be certain.

Do not run multiple antivirus programs at the same time. They will conflict with each other and slow your computer. If you want to use a different scanner, uninstall your current one first, restart your computer, then install the new one.

Key Takeaways

  • Windows Defender, built into Windows 10 and 11, can scan your entire system for free without installing additional software.
  • A quick scan takes 5 to 15 minutes and checks the most common hiding places; a full scan takes 30 minutes to several hours but checks every file.
  • Malwarebytes and Kaspersky are reliable second opinions if Windows Defender finds nothing but you still suspect an infection.
  • If a scan finds a virus, Windows Defender will quarantine it automatically, but you may need to restart your computer for the removal to complete.
  • Running two antivirus programs at once will slow your computer and cause them to interfere with each other.

Using Windows Defender for a complete scan

Windows Defender is your first choice because it is already on your computer and does not require you to read anything. To start a full scan, open Windows Security from the Start menu, go to "Virus & threat protection," click "Scan options," select "Full scan," and click "Scan now." Your computer will begin checking every file and folder.

During the scan, your computer will run slower than normal. You can use it while the scan is happening, but opening large programs or files will make the scan take longer. If you want the scan to finish faster, close other programs and let the scan run uninterrupted.

When the scan finishes, Windows Defender will show you a summary. If it found threats, it will tell you what they are and whether it quarantined them (moved them to a safe location where they cannot run). If it says "No threats found," your system passed the check. If it found threats but could not remove them, you may need to restart your computer and run the scan again.

When to use a second antivirus scanner

If Windows Defender finds nothing but you still think your computer has a virus — because it is running slowly, showing pop-ups, or behaving strangely — run a second scanner as a second opinion. Malwarebytes and Kaspersky both offer free scan-only versions that do not interfere with Windows Defender.

read Malwarebytes from malwarebytes.com or Kaspersky from kaspersky.com. Install it, open the program, and click the button to scan. These programs often catch things Windows Defender misses, especially newer or less common malware. If either one finds something, follow its instructions to remove it.

Do not install both Malwarebytes and Kaspersky at the same time. Use one, let it finish, then uninstall it before installing the other. This prevents them from conflicting with each other or with Windows Defender.

What to do if a scan finds a virus

If Windows Defender or another scanner finds a threat, it will usually quarantine it automatically — meaning it moves the infected file to a locked folder where it cannot run or spread. The threat is contained, but the file is still on your computer. You can then choose to delete it permanently or restore it if it was a false alarm.

After a scan finds and quarantines a threat, restart your computer. Some malware runs in memory while Windows is open, and restarting clears that memory and prevents the malware from running again. After you restart, run the scan one more time to confirm the threat is gone.

If a scan says it found a threat but could not remove it, the malware may be blocking the removal. Restart your computer in Safe Mode — a special startup mode where Windows loads only essential programs — and run the scan again. In Safe Mode, malware has less ability to protect itself. To enter Safe Mode on Windows 10 or 11, restart your computer, and as it boots, press F8 or Shift+F8 repeatedly until you see the boot menu, then select "Safe Mode."

Signs your computer might have a virus even if scans find nothing

Sometimes a computer shows signs of infection but scans find nothing. This can happen if the malware is very new, very well hidden, or if the slowness is caused by something other than a virus. Before assuming you have an undetectable virus, check a few other things first.

Open Task Manager by pressing Ctrl+Shift+Esc. Click the "Processes" tab and look at the "CPU" and "Memory" columns. If one program is using most of your CPU or memory, that program is slowing your computer, not a virus. Close it or uninstall it. If no single program stands out, your slowness may be a hard drive running out of space, a failing hard drive, or a virus that scans cannot find.

Check your startup programs next. In Windows 10 and 11, open Task Manager, click the "Startup" tab, and look at the list. Disable any programs you do not recognize or do not need to start when your computer boots. Startup programs slow your boot time and can be a sign of malware, but they are often just programs you installed and forgot about.

Preventing viruses between scans

Scanning finds viruses that are already on your computer, but prevention stops them from getting there in the first place. The most common way people get viruses is by opening email attachments or downloading files from untrusted websites. Do not open attachments from people you do not know, and do not read files from websites that look suspicious or ask you to disable your antivirus.

Keep Windows updated. Open Settings, go to "Update & Security," and click "Check for updates." Windows updates patch security holes that malware uses to get in. Updates happen automatically on most computers, but checking manually ensures you have the latest protection.

Use strong passwords on your accounts, especially your email and your Windows login. If someone gets your password, they can install malware on your computer remotely. A strong password is at least 12 characters long and includes uppercase letters, lowercase letters, numbers, and symbols.

Frequently Asked Questions

How often should I scan my computer for viruses?

Run a quick scan once a month if you use your computer normally. If you read files frequently, visit unfamiliar websites, or open email attachments often, scan every two weeks. If a scan ever finds a threat, run a full scan when ready and then scan again weekly for a month to make sure the infection is completely gone.

Does Windows Defender slow down my computer?

Windows Defender runs in the background and uses some CPU and memory, but the slowdown is usually small — most people do not notice it. If your computer feels slow, the slowness is more likely caused by too many startup programs, a full hard drive, or a failing hard drive than by Windows Defender itself.

What is the difference between quarantine and delete?

Quarantine moves a file to a locked folder where it cannot run or spread, but the file still exists on your computer. Delete removes the file completely. If an antivirus quarantines something by mistake, you can restore it. If you delete it, it is gone. Always let the antivirus quarantine first, then delete after you are certain it was actually a threat.

Can I scan my computer while I am using it?

Yes, but the scan will take longer. Close large programs like video editors, web browsers with many tabs, or games before you start a full scan. The scan will finish faster if your computer is not doing anything else at the same time.

What if I think I have a virus but I am afraid to restart my computer?

Restarting is safe and often necessary to remove a virus completely. Malware often runs in memory while Windows is open, and restarting clears that memory. If you are worried about losing work, save everything first, then restart. After the restart, run a scan again to confirm the virus is gone.