Out of the past 8 times I've seen this problem, 3 were caused by a virus, and 5 were caused by spyware... be sure to follow all of this advice and investigate both!
Try
http://housecall.trendmicro.com/
or search google for an online viruschecker (try Panda).
Also, be sure to download spybot, install it, UPDATE IT, and run it.
Finally, download Ad-aware, and try that too (updated)
http://www.lavasoft.de/support/download/