Windows Defender keeps turining on/off (Windows 10)

After removal of a 3rd party antivirus, the built-in Windows Defender anti-virus program just kept turning on and off. After trying several registry cleaners and reboots (to remove leftovers of the previous anti-virus program), the situation still was the same.

The proper way to fix this is:

1. Press Windows key + R. This will open Run.
2. In Run dialog box, type 'regedit' and hit enter. This will open Registry Editor.
3. In Registry Editor, navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender.
4. If you see a registry entry named DisableAntiSpyware, then change its value to 0. If you don't find this registry key then add this it. To do that, right click on Windows Defender key and go to New > DWORD. Give this DWORD name 'DisableAntiSpyware' and value 0. Registry will then look like this:
   

After few seconds, the Windows Defender service will start and remain active.