"Unexplained computer behavior may be caused by deceptive software"
http://support.microsoft.com/kb/827315
If you suspect your computer is infected, follow these steps:
1) If you have anti-virus and and anti-spyware programs already
installed on your computer, update the programs and scan your computer.
With luck, the programs will detect and remove the problem.
2) If the scan doesn't produce results, contact the program's technical
support. Most of the major anti-virus/spyware programs will help you to
remove an infection that their software did not detrect.
3) Scan your system with /several/ of the better online scanners
(Kaspersky, Eset, Trend Micro, Panda, Sophos, Symantec etc.)
4) Submit a HiJack This log to one of the expert web sites that analyze
these logs. To find HiJack this, look here:
http://www.spywareinfo.com/~merijn/programs.php
5) Finally, if none of the above remove the malicious software, it's
time to show the computer to a professional.
"So how did I get infected in the first place?"
http://forums.spywareinfo.com/index.php?showtopic=60955