"This app has been turned off and isn't monitoring your computer...use Action Center to check that app's status
" is typically the message provided when attempting to run Windows Defender on a system where a third party anti-virus is installed.
Go to Control Panel > System and Security > Action Center and you should see a red alert indicating "Spyware and unwanted software protection (Important) Windows Defender is turned off [Turn on now].
Click the "Turn on now" button and the Windows Defender screen should turn green and say "Your PC is being monitored and protected...Real-time protection: on.
" Please be patient as sometimes it may take a while for Windows Defener to turn itself on.
Click the "Update" tab to view the virus and spyware definition update status indicating when virus and spyware definitions was last created. If it has been a while you should see a message indicating "Your virus and spyware definition haven't been updated lately...You should update them now to help protect your PC.
Click the "Update" button so Windows Defender can download the latest definitions. After completing the update, the status will change to "up to date"
" and allow you to perform a scan.
If that does not work, see these suggestions...
The last two users in this topic
posted alternative solutions for taking Full Permissions Control To Edit Protected Registry Keys which worked for them. Always Create a New Restore Point
and back up the registry before making any changes
If this still does not work, you may have to switch to another anti-virus solution or:Repair Install Windows 8 (refresh and reset)How to refresh, reset, or restore your PC