Windows defender has stopped working

My OS is 'windows 8.1', For the last few days windows defender has not been working, I have done 'System Restores' to dates when every thing seemed to be working, but it has not solved the problem. I did a search for windows defender, and I got the following error message:

Security Client

An error has occurred in the program during initialization. If this problem continues, contact your security administrator.

Error code 0x80073afc

I shall be grateful for help in solving this problem.

Thanks
Preet