I always use antivirus on windows XP and windows 2000. But is antivirus needed on Windows 2003, when it is sitting on datacenter, nobody is playing games or downloading binaries from internet.? I never heard any antivirus software for Linux or FreeBSD. If needed, which antivirus is the best choice for Windows 2003?
As for firewall, is the builtin 2003 firewall good enough?
I would say that if you use Windows purely as a server and in NTFS it should be ok without anti virus. I have seen users use their server for sending/receiving emails in Outlook, surfing the Internet, this can be risky and will need an anti virus.
But for the peace of mind, try Clamwin.com, a free anti virus software for Windows.
For Windows 2003 firewall, I guess it is basically similar to the normal software firewalls on the market but with less features. Proper patching, maintenance, stopping of unneeded services and blocking of unneeded ports using TCP filtering is normally sufficient.