Sounds to me like it could be an adware program installed on your computer that runs everytime you open your browser.
I'd recommend scanning your PC at housecall.com, and getting a copy of Lavasoft Ad-Aware or Spybot Search & Destroy in order to eliminate the adware from your system. If you're using Kazaa, then I'd suggest uninstalling it and getting a copy of Kazaa Lite. The normal version of Kazaa has adware on it.