Sounds to me like a bad case of spyware, or something like that. Or maybe way underpowered? (How old is the PC?)
If you haven't checked for spyware, that would be my first try. Go to www.lavasoft.com and download, install, and run AdAware. It's free. Be sure to choose the option to look for the latest update files the first time you run it, before doing the scan.