Try both of these. After they are both done scanning and repairing, reboot your PC and run them again. Some viruses or ad trojans can pop back into existence after rebooting. Those can be difficult to get rid of. If you have one, you can google the name of the file or virus to find ways to delete it.

AVG free edition Free virus scanning.
This will find a big list of "tracking cookies". advertising servers use these to keep track of which sites you visit that use their ad services. It's a privacy thing to delete them, but they aren't any danger to you. Any viruses or spy bots will be listed near the bottom of the list, and not be called "cookies"

Spybot search and destroy. Free (or donate) ad hijacker and spy bot cleaner.