Virusprotection@home
For virus protection in private (individual, non-commercial) fields, H+BEDV Datantechnik GmbH offers their programm AntiVir for free.
Die free version runs on Windows 95/98/Me & NT/2000/XP.
The professional (non-free) version runs on Linux too
(if you intent to run your server as a file-server or mail-gateway).