Try this one first:- How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
Sometimes works in these situations.
System Restore would work of course, rolled back to days ago.
if it has been corrupted by say virus, you would be rolling that back too?
So try the boot see if that works first.
If not :-
First, to assist in the elimination of possible virus and/or malware.
I would ask you to please use your Anti-virus full scan program facilities, making sure, you have updated the software first. Please advise us of the results.
If you do not have any anti-virus loaded please go to this link and download Microsoft Security Essentials :- Microsoft Security Essentials | Protect against viruses, spyware, and other malware
Secondly, please download:- malwarebytes Malwarebytes
a Malware searching program, again, an updated full scan. Notify results please. Both these programs are Freeware