Your computer has one or more pieces of 'adware' or 'malware'. Use a scanning utility such as Ad-Aware or SpyBot Search & Destroy to remove it. (It's a good idea to run one or both of those on a weekly or monthly basis anyway.)
what browser are you using? When did it start happening? On just the main page or on a particular thread?
I have no problems using Safari 1.2.4 or IE 5.2 (both on Mac 10.3), or Lynx 2.8.5dev.7 (Linux). The last time I checked at work (sometime in the last week I think) with IE6 I didn't see any wierd popups there either.
Have you checked your PriusOnline profile to see if you have popups enabled for a new Private Message? (Do you have a new private message?)
Same problem here. ie 6.0. Started on turkey day. Medie Places box pops open with error message, "The page cannot be displayed". Ran all of my checkers with nothing found. All other boards ok. Any ideas?