Update: don’t use AVG anymore but its still a good tool.

This is a good virus scanner, and i have tried a lot!
Out of all the ones there are 3 i liked best:
NOD32, for the sheer speed of it
AVAST, quite a good scanner with a nice interface (skinned!) and a free version for personal use
AVG - more in this article

Note that i own copies of both mcafee and norton (from their utility packs which i regularly upgrade even though i dont use much of it) and chose to buy another virus scanner. Norton was just too resource hungry and obtrusive. Mc Afee was pretty good but when my year last expired I switched to AVG after having tested AVG free on a laptop.

Now I own the AVG SOHO edition which is 2 licenses for two years at a very nice price

They do a free version for personal use based on their old software. It still gets all the virus profile updates and if you dont have a virus scanner go and get this one.

The software uses very little resources when in backgroud scanning more, even with heuristics turned on. It watches very tightly over email streams - it has a pop proxy mode but even without that on it is so quick to scan files that as soon as your email scanner decodes the attachement to store it on disk the AVG window pops up.

It updates itself daily (if you configure it to) and just does it job well with very little need for intervention - it is the least obtrusive one i have tried.

Add to this that it is the cheapes of the good reliable virus scanner, esp. when you consider the license is valid for two years. http://www.grisoft.com or http://www.avguk.com