You either got infected with something or something in the registry is corrupt. You could do a System Restore if you knew the approximate date the problem started.
There is a way to do a repair on Win7 in order not to lose any documents, photos, etc. I've done it twice so far, because I experiment a lot, and it works completely. Just put in the disk and go through the regular process and chose the upgrade option.. Let it get all the updates when asked. You won't lose anything this way except some visual settings. Good luck.