You almost certainly have spyware that has hijacking capabilities. Some of these little programs track what websites you're at and try to match the subject matter with popups that are related. They also change the settings in your browser and Windows programs (called system registry).
Download "AdAware" and "Spybot S&D". I run both occasionally or whenever I have a problem and they are very effective and do not negatively affect the performance of my machine. Another powerful tool is "HijackThis" which will scan your registry when you ask it to and list any suspected modifications to the registry. It is possible to do damage to the normal function of your registry if you modify the wrong things though so be very sure before you tinker.
Finally, there are techies out there who will tell you which things to remove with "HijackThis" ("HT") if you send them a list of the HT results.
Ryan
June 2004