I use
NOD32 on one of my systems, and can vouch for the fact that it is very light on the CPU. I don't kid myself into thinking that it is the best at malware detection though. People will argue tooth and nail that NOD32 catches anything that is "in the wild", but when you happen upon a trojan on a newsgroup, or in that bit of freeware you wanted to try out, it really won't matter to you if that trojan is in the wild or not.
I hear that Dr. Web is also a good choice for being light on the CPU. I have no experience with Dr. Web, though, aside from its integration with Nero, which counts for nothing.
I would pose your question in the
Wilders anti-virus forum. Beware, though--those forums are rife with irrational product loyalty.
The ultimate suggestion is to try the few you think are good candidates, see how they do on testing sites like
AV Comparatives, and make up your own mind. (And if you ask for suggestions on Wilders, about 10 people will tell you this same thing anyway--after the 40 or 50 people who say "NOD32!" and "Dr. Web!", that is.)