i found two link that might help you
try it
https://www.vistax64.com/tutorials/218830-windows-defender-fix-unable-turn.html
https://social.technet.microsoft.com/Forums/en/itprovistaapps/thread/f5536f2d-aceb-41a5-beb6-92e5d3b2f73f
also,you can try those changes in safe mode