In order to fix the issue of Windows Defender Antivirus not turning on, you can try using DISM scan instead.
Further tip: If your Windows Defender can work normally, but you have a need to disable it, you can watch the following video.
If you have installed any third-party software on your computer, Windows operating system will automatically detect it and turn itself down.
Upgrade Windows OS Now, it is time for you to install the latest Windows operating system. Here is the guide: Step 1: In Windows 7, go to Control Panel and click Windows Update.
In Windows 8, go to Control Panel Sometimes, Windows Defender won't turn on in Windows 10/8/7 because it is turned off by your group policy.
An outdated Windows operating system may cause the issue of Windows Defender not turning on.
Some users reported that they had been able to fix the issue simply by installing the latest Windows update - latest signature updates are necessary for Windows Defender to keep your PC secure.
Finally, restart your PC and try launching Windows Defender again to see if it can be turned on for virus, spyware and other threats protection.
In order to let Windows Defender work properly, you need to enable certain services.
To put it simply, this issue is possibly caused by the installation of third-party antivirus software, a corrupted registry, software conflicts, and so on.
For Windows 7, Windows 8 and Windows 10 users, it is fairly common.
Thus, the first thing you can try is to disable your other security programs.