Antivirus for Win-XP with light footprint

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 on a light-duty antivirus program with a low overhead? (These people do not tend to surf "dirty" sites.)