I've used Trend Micro's anti-virus software for the last 2 1/2 years and have been very pleased. Server Protect for our 10 servers, OfficeScan for our 175 workstations and ScanMail for our exchange server.
The server downloads the virus profiles from the internet (mine is set to check for upgrades every evening) and then pushes the upgrades out to each of the clients. If you're running an NT network you can also remotely install the virus software on the clients.
For the 2 times I've needed tech support they were quickly available and solved the issue in a reliable and professional manner.
They've provided our non-profit with discount pricing.
They evern have a free online virus scan.
You can get more info at
http://www.trendmicro.com/en/home/us/smb.htm