Problem:

Net Express 3.1 is certified on Windows 95 (Windows 98 recommended), Windows NT V4.0 (Windows NT Service Pack #5 or later), Windows 2000, or Windows XP Professional.

Net Express 4.0 is certified on Windows 98, Windows NT 4.0 (Windows NT Service Pack #6a or later), Windows 2000, Windows Server 2003 or Windows XP Professional.

Net Express 5.0 is certified on Windows 98, Windows NT 4.0 (Windows NT Service Pack #6a or later), Windows 2000, Windows Server 2003 or Windows XP Professional and Windows Vista.

NetExpress 5.1 is certified on Windows Server 2008, Windows Vista, Windows Server 2003, Windows XP or Windows 2000

Resolution:

Net Express 4.0, 5.0 and 5.1 are certified on Windows Server 2003.