Ask Slashdot: Light-Footprint Antivirus For Windows XP?

New submitter Bauermlb writes "I service computers for retired folks in my community, often older machines with modest speed (2 GHz Centron) and modest memory (512 MB). Adding AVAST to one of these machines slows it to a crawl. Any recommendations for a light-duty antivirus program with a low overhead? (These people do not tend to surf 'dirty' sites.)"