Most likely your computer has been infested with malwares. You'd need to get rid rid of them.
Two best freewares are available. I'd suggest you to use both. Please do not run both programs at the same time. Use one, when finished, close it and then run the other. Here's the links for direct download. Good luck:
Ad-aware 6.0 build 181 >>
Spybot - Search & Destroy 1.3 >>