AVG free edition virus scanner.
http://free.grisoft.com/freeweb.php/doc/2/But he's right that you may have problems installing it into a corrupt system.
I've had fairly good luck with housecall myself.
If your running XP your options are limited.
If your running 98SE you could always download a dos based scanner, burn it to a CD, boot with your 98startup disk. Do a dos scan. Then install AVG, and scan again.
Last, I've had Ad-aware SE picking up things that others have missed lately.
So you might want to try that first.