See if the scans Grif advises in http://forums.cnet.com/7726-6132_102-5349457.html?tag=posts;msg5349457 find it.
There's more than antivirus.
A friend of mine asked me for assistance with a problem he is having with his PC. Every time his PC goes into sleep mode, his default web-page changes to "mysearchresults.com" or something like that (some no-name search provider).
I changed his default back to Google, and the problem persisted. I ran a virus check, a scan for broken shortcuts, a registry defrag, and checked the Microsoft website for updates. I also checked his installed programs, and IE add-ons, and nothing looked out of the ordinary.
What could possibly be causing this?