Kerim, this happened for me yesterday, or Tuesday, after updating Windows.
Repair does not help for me.
I can't find any other infection or corruption... I scanned with Ad-aware, Spybot, Ewido, Avast, AVG, Antivir... a very deep scanning of the system. Nothing.
After a boot, avast! worked again...
I think if you just run the setup.exe file 'over' the old installation, it could work