Please - this is not a BES thing - Go look it up at a MS website. Exchange 2000 will not run on a Windows Server 2003 machine. Has nothing to do with BES. Exchange 2003 will run on a Windows 2000 Server but not the other way 'round. -->
http://support.microsoft.com/?kbid=321648 You Cannot Install Exchange 2000 on a Computer That Is Running Windows Server 2003 <--