There are few Free Antivirus software available for Windows, most are for Windows Home Users Only. Since Windows 2003 is server OS, most of these free Antivirus software do not run on Windows 2003. I use Windows 2003 on my desktop, i like it much better than Windows 2000 Pro. I don’t like Windows XP bacuse of its colors, i have installed it two times, but uninstalled it, found i can make it look like Windows 2003 by select classic theme, but i like using Windows 2003 on desktop.
ClamAV is a Free Antivirus software available for Linux and FreeBSD. Most of the Cpanel servers have this default as it can easily installed through Cpanel Addons.
Good news for Windows 2003 users is ClamAV is now ported to Windows, its free to use.
I may wait some more time to get it more stable, by this time, mail servers will add support for this Free Antivirus.