If you're running AIM, it could come from there, my roommate had the same problem and that's where it came from. I have no idea how to turn it off though, check the options or installing something like DeadAIM.
If it's not coming from AIM, I have no idea

Maybe you have some sort of adbot installed on your computer, go to www.lavasoft.de and download the Ad-aware software

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get adaware 6 and spybot-seach and destroy. it gets alot more crap that adaware misses.http://www.safer-networking.org/

Yep, Adaware should get anything and everything out! Make sure you run the in-depth scan.

Thanks for the responses guys -- I'll run an Ad-Aware in-depth scan tonight as I sleep