I think that you will have a difficult time finding something that works. There are free antivirus solutions but they are targeted towards consumers, not businesses, and consequently they typically do not install or function on server operating systems. Instead they usually have server or business editions that are available for a fee, and they use the free consumer edition to get their foot in the door and get a larger installed base for threat detection. I think that the assumption is that if you can spend $800 or more on a server OS license then you can afford to license an antivirus solution.
________________________________________
CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCSE:Security 2003
MCITP:Server Administrator
MCITP:Enterprise Administrator
MCITP:Virtualization Administrator 2008 R2
Certified Quest vWorkspace Administrator