For two months, my Windows Defender was completely off and could not be turned off. I worked with a forum where hundreds of others had the same problem and error code.

Now after getting a notice of a driver update from Dell, Defender appeared once again.  It has updated now and scanned (apparently - done manually), but when you try to see if has detected anything you get a message "the class is not configured to support elevated activation."

For help see here

http://answers.microsoft.com/en-us/protect/wiki/protect_defender-protect_start/problems-starting-windows-defender-in-windows-881/808253bb-db89-4db9-a4e5-1c91a86489e9

http://www.thewindowsclub.com/unable-to-turn-on-windows-defender

https://www.winhelp.us/configure-windows-defender-in-windows-8.html

etc

M.