Not sure when this started happening, but I can't seem to install programs anymore on my machine. I ran the Microsoft FixIt tool, which helped people with similar situations, but it didn't find anything wrong with my computer. Does anyone know what else I can try?
well, I think your pc can be infected with virus problem. Boot into Safe mode by pressing F8 during to boot and choose Safe mode. Try if you can install a small+simple software in safe mode. But i suggest you do a full virus scan with a good antivirus software.
p/s: the quick way is re-install your windows 7
Hope this help