If you are using AVG anti-virus and still on version 8, you have no doubt seen these messages numerous times, urging you to move to version 9.
My own tests of version 9 showed it to be even more resource-intensive than version 8, but after a quick search at SourceForge I found ClamWin which is a free open-source virus scanner. So far the results have been favorable.
I am running Windows 2000, XP Home and Pro, and Server 2003, and ClamWin is compatble with all of those OS's. ClamWin runs a single Windows process ClamTray, as opposed to four processes with AVG (avgnsx, avgrsx, avgtray, avgwdsvc) and it includes integration with Internet Explorer and Outlook.
At this point I have ClamWin running on the Server 2003 and Windows 2000, and I plan to start testing on the XP machines in the near future.