Sounds like you've picked up spyware somewhere. Here is a great tool to scan for and get rid of stuff like that:
Malwarebytes Anti-Malware - Free software downloads and reviews - CNET Download.com
If you have toobars by Yahoo, MSN, or Google, they can do that too, and it's not only annoying, it tracks your internet surfing too. BIG violation of privacy, IMHO.
Good luck
!