Trend Micro PC-Cillin Internet Security Its been updating a lot these days..in fact twice a day it updates its virus and other definitions!
MC afee the best but heavy if you like to use tiny but powerfull you can use ansav check at ansav.com hope usefull
usta use kaspersky till it expired i liked it using mcAfee security center on my laptop and ca internet suite on my desktop! I like em both so far! may put ca on my laptop also being i paid for 2more installs!
I don't really use Anti-Virus due to the memory usage although from past experience AVG is really good. The only thing is it updates as soon as you log in which is pretty annoying and slows the PC down for a while.
I use NOD32, one of the best antivirus programs around, requires small resources unlike some... btw those in the uk if you get this months pc advisor you can get kaspersky antivirus (another good one) with 12 months virus definitions for free