In my opinion, the best at the moment is Microsoft's own Microsoft Security Essentials which is free, updates automatically and can be set to scan in the background under processor resources you can limit. It's free from their website under the Security tab and you can get the file mseinstall64 for your system.
I don't know about specifically for server 2003, but I have Windows and I use Unthreat which is a free antivirus and it does a great job of protecting my computer. It's easy to configure to my own preferences and doesn't slow down my computer when working. I recommend it.