I wouldn't use any free AV on w2k3 server
w2k3 is not a cheap OS to use free AV. it might cause w2k3 crash more often
I'm absolutely fascinated by this comment, especially as you appear to have chosen a free mail server over one of the commercial offerings, such as Exchange. What does the cost of the OS have to do with choosing software? Even more importantly, why do you think free, and perhaps even cheap, software may cause the OS to crash more often?
I use 4 different AV products on a range of operating systems, 2 free and 2 commercial. Only the non-free ones (Sophos and McAfee) have ever crashed on my. ClamAV, including the ClamWin variant, works perfectly on any OS it supports.
The most stable software I use is all Open Source (read: free). On the other hand, highly unstable software, such Microsoft Office and most Adobe products are absurdly expensive.
You can ignore the wild claims made by parties such as Microsoft. When talking about software in general there is no direct correlation between the quality and stability of software and its cost, nor has there ever been in the 35 years I've been in the game. Needless to say, there are numerous exceptions on both side of the fence.
Reality: Free software is rarely rushed out just to meet a deadline, meaning there is more time to develop and debug. Commercial software is seldom well debugged because it IS rushed out to meet a deadline.