How to Reset Microsoft Defender in Windows 11

If restarting your PC doesn’t work, reset Windows Security in a few seconds.

Windows Defender not opening or working correctly could mean you can’t edit any of its settings in Windows Security, or malware can run amok on your system because you lack the proper security. Below are several reasons why Windows Defender doesn’t open and how to get it to work again.

Why Windows Defender Won’t Open in Windows 11

Here are some examples of when Microsoft’s Defender app won’t open or work properly:

• There’s a temporary memory-related issue where the app won’t scan for malware.
• Windows Security usually opens but then promptly closes a few seconds later.
• Another software program is “fighting” with Windows Defender, throwing error messages.
• You see an error that says, “You’ll need a new app to open this Windows Defender link.”
• You recently installed or uninstalled another virus protection program, and now Windows Defender won’t open.

How to Fix Windows 11 Defender Not Working

There are several things you can try, but follow this list in the order it’s presented to address the more accessible solutions first:

• Restart your computer.
  This is a very common first step when troubleshooting anything in Windows, and it might just be all you have to do to get Windows Defender working again.

• Turn off Windows Defender, and then turn it back on.
  If it was off, that article shows you how to turn it on.
  Microsoft Defender won’t catch active malware unless you have Real-time protection turned on.

• Repair or reset Windows security. A repair will have Windows attempt to fix whatever is going on, and a reset will erase all the app’s settings and start over as if Windows Defender was just installed.
  Follow that link for two methods—one uses Settings, like in the image below, and the other uses a PowerShell command that’s helpful if you can’t open Windows Security.

• Update Windows. This is how Microsoft delivers bug fixes, so an update you’ve yet to install could address the Windows Security issue.
  You can try an offline Microsoft Defender update if that doesn’t work. Choose the latest version from that list, likely called Update for Microsoft Defender Antivirus antimalware platform.
  https://www.catalog.update.microsoft.com/Search.aspx?q=defender

• This step is for when your problem is so specific the only issue you’re running into is you can’t clear out the Protection history screen in Windows Security, or you’re told a threat was found, but there’s nothing to clear out.
  To address this, open the following folder. One easy way to do this is to paste this path into the Run dialogue box (WIN+R):
  [ C:\ProgramData\Microsoft\Windows Defender\Scans\History\Service\ ]
  Select everything in the Service folder (Ctrl+A), then press Delete to clear it.

• Temporarily disable any other antivirus programs you have installed. How this works is different for every app, but doing so should give you some time to see if Windows Defender’s troubles were due to a compatibility issue with the other antimalware tool.
  If you find the other program is to blame, or you suspect it might be, but disabling it didn’t help, use a program uninstaller tool to completely delete the software.
  If you still can’t start Windows Defender, you can delete a couple of registry keys to re-enable it. This might happen after uninstalling another antivirus program. Open this key, and delete these registry values: DisableSpyware and DisableAntivirus.
  [ HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender\ ]

• Run the SFC /scan now command to repair system files. This will invoke the System File Checker tool to see any problems with protected Windows files and then replace them if so. 

• Use Reset This PC to reinstall Windows 11. Although this will reinstall Windows and reset all your programs to their factory default state, that’s precisely what you want.
  This is a sure fix for whatever problem is affecting Windows Defender. Just wait until you’ve tried all the above before completing this drastic step.