WHat KInd of VIrus is THis?

I use Windows XP SP2. It somehow got infected with a virus I've never encountered before. Not really destructive but very very annoying. Everytime I try to open websites like "orkut" and "youtube", internet explorer would close immediately and a pop message that says "Orkut/youtube is banned, you fool!". Mozilla Firefox, on the other hand, cannot be used at all now. Window would immediately close as soon as you open it. I also used Vista in the system, but it's not infected. I've used Avast anitvirus to scan the system, but is of no use. Can you Please Help?