Have you tried a full system scan with an up-to-date antivirus program? Or, if you don't have one installed, there are free online scans at
Panda and
Housecall.
If you know the system is clean of viruses, have you scanned for and removed all spyware? If not, scanning with both
Spybot and
Ad-Aware should find and be able to get rid of 99.9% of the spyware out there.
Finally, since neither AV or anti-spyware programs are always effective for indentifying or removing trojans, a good free online trojan scan is on
Windowsecurity.com.