I agree with the NOD32....It is very light on resources and is an excellent scanner. Picks up viruses, Adware, some Trojans and it now just came out with Rootkit detection and removal!
You pay a moderate amount for it (I think it's $39), but it's well worth it...I haven't had any problems with malware since I started using it two years ago. It has caught a few drive by downloads and quarantined them before they executed. I've tried many scanners and this has been the best so far. They also have a 30 day free trail!
NOD32 30 day trial