Usually, when Windows Defender is not working, the best way to fix it is to perform a repair upgrade.
See steps how to do that:
http://answers.microsoft.com/en-us/insider/wiki...
Other steps you can also try:
Try these first to see if they work:
Open Start, type: CMD
Right click CMD
Click Run as administrator
Type in at the prompt OR Copy and Paste these one at a time : (Hit enter after each)
Dism /Online /Cleanup-Image /CheckHealth
Dism /Online /Cleanup-Image /ScanHealth
Dism /Online /Cleanup-Image /RestoreHealth
Also run the System File Checker utility:
Open Start, type: CMD
Right click CMD
Click Run as administrator
At the Command Prompt, type: sfc/scannow
This will check for any integrity violations
Restart your system