It could be some virus.
But i had this a while ago.
when i used regclean, the problem was gone

Also try these links
http://housecall.trendmicro.com/ (online virusscanner)
http://www.majorgeeks.com/ (lots of free programs against spy/mal/ad-ware
http://www.spywareinfo.com/~merijn/ (homepage of csShredder and HiJackThis)