Use ESET NOD32 Antivirus. It's the most efficient AV software on the market with the best detection (proactive!) and lowest resource utilization. It won the AV-Comparitives.org best AV product award.
Check it out!
ESET's homepage is
http://www.eset.com. Try the demo free for 30 days. I did over a year ago and was amazed at the results - the server runs much more smoothly too. We had Symantec before that and then we tried a bunch of others including McAfee, AVG, Bit Defender, Kaspersky, and others. NOD32 won hands down. Pretty darn cool.
I've been using version 2.7 on my Windows Server 2003 machine, but they just came out with version 3.0. I'm testing that to see how it works.
P.S.: They have domestic support for all countries they do business in, so no talking to India if you're from the US or UK!