Not much of an expert on these things myself, but if you antivirus can't do anything about/doesn't flag it, then a couple of other recommendations off the top of my head that may work:
Check for browser plug-ins you might not have authorized (not sure your browser but somewhere in settings there should be a list of installed plug-ins that you can manually remove - get rid of anything that looks fishy.
Clear out your cookie cache.
Go through Add/Remove Programs (or if you have CCleaner already I usually go through that) and see if there's anything there or in your registry that looks fishy.
Not sure any of that would help, but that'd be my best guess.