Hello Lex,
Most Server AV's will require a purchase of 5 Licenses or more. What version of Server 2003 are you running? Enterprise, Small Business Server? Need some info to help with results.
TrendMicro, Eset NOD32 Business, among others will work.
Purchase Antivirus and Security Software at ESET. Buy Antivirus Programs and More.
Eset for NOD32 Business has a option for 1 License. That might be your best option. But from the looks of it that is only for the noremal NOD32 not the business.
Cheers,
Mak