AVG (Free or Pro) has minimal impact on system and has a reasonable “hit rate”
F-Prot claims their scan engine uses very low system resources and can runs under Windows 2000, 2003 or XP.
Yoggie produce USB and Ethernet plug-in hardwrare solutions which will protect one machine or a whole network of machines – I am researching at present for my own use – they not only provide AV protection but also firewall and other security functions. They seem to be reasonably priced (from a UK standpoint) and reportedly take a big load off the PC processors. (I have to protect three laptops on the network).