I guess the question is what caused this? There are reports that adware infestation cause this. Other reports are that it is caused by the PRORAT-P Trojan. the HWBOT.E Worm or MYTOB.BS Worm.
Have you tried downloading and running Ad Aware? It is free at
www.lavalys.com . Also, Spybot and AVG Free AntiVirus could come in handy about now? They are free too, just google for them.
I am guessing that you do not have Adware/spyware/virus scanners in place, current and being used. If you are using Windows, you do not have the luxury of this.