Windows Defender finds malware, but not all of it
Windows Defender is the built-in antivirus program that comes with Windows 10 and Windows 11. It scans your computer for malware — viruses, trojans, spyware, and other threats — automatically in the background. It runs scans when you're not using your computer, checks files when you download them, and monitors programs as they run.
The catch is that Windows Defender is a basic protection layer, not a complete shield. It catches common, well-known threats well. It misses newer malware, sophisticated attacks, and some types of spyware that other security programs catch. If you download files from risky websites, use public WiFi often, or open email attachments from strangers, Windows Defender alone may not be enough.
Key Takeaways
- Windows Defender scans automatically in the background and catches common viruses and malware without you having to do anything.
- It is less effective against new malware, targeted attacks, and some spyware compared to paid antivirus programs.
- You can run a manual scan anytime by opening Windows Security and selecting "Virus & threat protection."
- Windows Defender works best when combined with safe browsing habits — not clicking suspicious links, not opening unexpected attachments, and keeping Windows updated.
How Windows Defender scans your computer
Windows Defender runs three types of scans. A quick scan checks the most common places malware hides — your Downloads folder, temporary files, and running programs — and takes a few minutes. A full scan checks every file on your hard drive and can take an hour or more. A custom scan lets you pick specific folders to check.
By default, Windows Defender runs a quick scan every week when your computer is idle. You don't have to start it yourself. If Windows detects a threat during any scan, it quarantines the file — moves it to an isolated folder where it can't run — and tells you what it found.
You can also turn on real-time protection, which is on by default. This watches files as you download them and programs as they launch, catching threats before they can do damage. If you disable real-time protection, Windows Defender stops scanning until you turn it back on.
What malware Windows Defender actually catches
Windows Defender is good at finding malware that has been around for months or years. Microsoft updates its malware definitions regularly — the list of known threats — so it recognizes viruses, trojans, and spyware that are already documented. If a piece of malware is widespread and well-known, Windows Defender will catch it.
Windows Defender struggles with zero-day exploits — brand-new malware that nobody has seen before and that Microsoft hasn't added to its definitions yet. It also misses some spyware designed to hide from antivirus programs, and it is less effective against ransomware that uses advanced encryption. Malware authors specifically design some threats to evade Windows Defender because it is so common.
In independent tests by security labs, Windows Defender typically catches 95 to 98 percent of known malware. Paid programs like Norton, McAfee, and Bitdefender sometimes catch a few percentage points more, especially on newer threats. The difference matters most if you regularly visit risky websites or download files from untrusted sources.
How to run a manual scan with Windows Defender
You don't have to wait for the automatic weekly scan. You can scan your computer anytime in a few steps.
- Click the Windows Start button and type Windows Security. Open the app that appears.
- Click Virus & threat protection on the left side.
- Under "Current threats," click Scan options.
- Choose Quick scan to check common locations (takes 2 to 5 minutes), or Full scan to check your entire hard drive (takes 30 minutes to an hour).
- Click Scan now. Windows will show you the progress and tell you when it's done.
If Windows Defender finds malware, it will quarantine it automatically. You'll see a list of what was found and can choose to remove it permanently or restore it if it was a false alarm.
When Windows Defender is not enough
Windows Defender is designed for basic protection — the kind most people need if they browse normally and don't download files from sketchy websites. If your situation is different, you may want additional protection.
Consider a paid antivirus program if you download files often from the internet, use public WiFi regularly, or work with sensitive information. You should also add extra protection if you've had malware before or if you visit websites known for hosting malware. Paid programs update their threat definitions more frequently and catch newer malware faster than Windows Defender.
You can run Windows Defender alongside another antivirus program, but not two paid programs at the same time — they will conflict with each other and slow your computer down. If you install a paid antivirus, Windows Defender automatically turns off to avoid the conflict.
Keeping Windows Defender working well
Windows Defender works best when Windows itself is up to date. Microsoft releases security updates regularly, and Windows Defender's malware definitions update automatically several times a day. You don't have to do anything — it happens in the background.
Check that real-time protection is turned on by opening Windows Security, clicking "Virus & threat protection," and looking for the green checkmark next to "Real-time protection." If you see a warning icon instead, click it and turn real-time protection back on.
You should also keep Windows itself updated. Go to Settings, click "Update & Security," and check for updates. Restart your computer when Windows asks you to. These updates patch security holes that malware could use to get in.
Frequently Asked Questions
Does Windows Defender slow down my computer?
Windows Defender uses some processing power, but most people don't notice it. It scans when your computer is idle so it doesn't interrupt your work. If you feel your computer is slow, a full scan might be running in the background — you can pause it or schedule scans for times you're not using the computer.
Can I turn off Windows Defender?
You can turn off real-time protection temporarily, but Windows will turn it back on after a few hours for security. If you install a paid antivirus program, Windows Defender turns off automatically. Don't leave your computer without any antivirus protection running.
What should I do if Windows Defender finds malware?
Windows Defender quarantines malware automatically, which stops it from running. You can then remove it permanently. If you think it was a false alarm, you can restore the file, but only do this if you're sure the file is safe.
Is Windows Defender better than paid antivirus programs?
Windows Defender is good for basic protection, but paid programs often catch more new malware and offer extra features like password managers or VPNs. For most people, Windows Defender is enough. If you download files often or visit risky websites, a paid program may be worth it.
How often should I run a full scan?
Once a month is reasonable if you browse normally. If you download files frequently or think your computer might be infected, run a full scan weekly. Windows Defender's automatic weekly quick scan is usually enough for everyday use.