Run a virus scan, and make sure you have something thta detects spy wear...if you dont, try download.com and find spybot search and destroy, gets rid of all those anoying bugs put in your computer that track you and what your doing, (most websites automatically do add one of these to you)!
Spywear can track all details like bank details and pass words! so always run a search regulaly!
It could be a trojon virus as well. Use a really good scanner that can detect anything amiss, even if it means you can't use your computer for an hour. Hopefully that'll solve it. If not, try a repairing program if your computer has it that'll detect any damaged files.
That exact some thing happened to me on my old computer!! I know how you feel. So many blank windows popped up the computer froze!! It only happened with Explorer so I started using Firefox.. Firefox never did that. I don't think it's a virus.. Both my anti-viruses didn't detect it.