I was disappointed by AVG when it royally screwed up my ability to send email, so I went for the next most efficient one on SchadenTech's list, which is F-Prot. It also uses very little resources, and the SchadenTech roundup suggests that its detection rate is better than AVG. Been running it for about five months now, and am very satisfied ... it feels totally transparent, except when it is putting the kibosh on a would-be infection (as it did very effectively when I visited the wrong website a few weeks ago). F-Prot isn't free, but sometimes you don't get what you don't pay for.