I never run AV on servers. It's not needed if they are properly managed and secured. As long as your admin isn't checking email on it or surfing the web, your server will be fine. Those are the vectors that 99.99% of viruses get in. Most antivirus / AV sales are made on poor admin knowledge and scare tactics. Also, make sure any services exposed to the internet are only as open as they need to be (port 443, 80, etc), and running behind a proper firewall. Run the windows firewall all the time as well, only open ports you need, and dont let non-admins login to the server. I've never had issues. If you must run AV for some reason, then I'd choose Microsoft Forefront Endpoint Protection - it's the server version of Security Essentials (which is for workstations). But do run MSSE (or other) on workstations - that is a must. Your servers are not going to be the point of entry if properly managed - the workstations will be (the ones where the users click bad email links - oops and navigate to questionable web sites).
Adding: Why are you running Server 2003?...you do know it's 2012 and that's a very old OS. Server 2008 R2 is minimum we run - it's far more secure by default. If you are worried about security - I'd upgrade your OS first.