If you're having the problem here, it could be coding set into someone's sig images. If you get Pop-Up Stopper, it should get them out of your way. You can get it for free at http://www.panicware.com
While using IE some spyware my sister had inadvertently downloaded kept opening context sensitive popups. Those things are a plague.
Ever since I use Opera I stopped having popup problems (built-in popup killer and worm resistant email client).
If you are on a Mac, Safari kills all pop ups. easy as a button click to turn them on or off.
If you find that the ads are popping up when you enter a specific thread here on the TMF please point me at it. We've had the issue of popups embedded in sigs before. I'll zap it.
It's probably a spyware program that was packaged with some other program. Use the free Adware prgram from lavasoft.com to erase any of these annoying progs.