I personally use Panda, which isnt free (although they have a free online scanner)... I find it NOT to be a resource hog. Norton is horrid when it comes to resource usage.
CA's EZ Trust AV his known to be pretty weak in virus detection. With them, youre pretty much getting what you pay for.
AVG tends to be the favorite among free AV's on the net. Id use it myself if I wasnt pleased with my Panda.