Server 2003 is mostly used in business environments. There is no free anti-virus that I know of for business use. Free software is usually licensed for home use (non-profits) only.
oh sure there is ! ClamAV is an open source (GPL) antivirus engine designed for detecting Trojans, viruses, malware and other malicious threats. on windows server2003 been using it for 15 years, http://www.clamav.net
Server 2003 is mostly used in business environments... the key word here is MOSTLY! There are plenty people who have Server running at home (which sysadmin hasn't?!) We have to keep up with technology and we're probably a rare breed that actually cares about security! These answers always madden me...