First thing to do: Try restoring to a previous restore point.
If you tried that and it did not work, then...
It looks like you have a corrupt essential system file. Not a nice situation.
Usualy, the only solution is using the install DVD of the operating system and perform a system repair.
DO NOT do a factory restore! If you do, you'll loose ALL your files, some of those for ever as they are overwriten by the process.
If you don't have the install DVD, you can get one as an ISO file from most torrent sites.
Using such a copy to perform a repair or replace a lost copy IS LEGAL! As long as you have the original product key.
Electro, July 2015