For a verrrry long time my Windows Defender has been off and everytime I try to turn it on it looks for definitions update only to say there are none and it fails. I’m finally getting around to fixing all the problems with my PC and so a quick google search found me the solution:
Click Start –> Run
Type “cmd” into the box.
Type the following command line: “winmgmt /verifyrepository”
The system may return a message saying, “WMI repository is INCONSISTENT”
If so, type this into the command line: “winmgmt /salvagerepository”
The first time you run this it will fail. (It will issue stop commands to the services causing it to fail. It might take a couple minutes for the services to shut down). Run the command again. You actually may have to run it 3 times before it finally runs and completes on its own.
Reboot your system.
Try turning Windows Defender on at this point if it doesn’t automatically turn on (though it probably will).
This will also ensure that Windows Defender detects the antivirus program you use (if you have one).