I run an Exchange Server 5.5 machine on a Windows NT 4.0 BDC. I want to migrate the Exchange 5.5 machine to an Exchange 2000 Server system. As an intermediate step, can I run Exchange 5.5 on a Windows 2000 DC?

The answer is a qualified maybe. A conflict exists between the Exchange 5.5 Directory Service (DS) and the Win2K Name Service Provider Interface (NSPI). Because the NSPI is part of the Global Catalog (GC) service package, you can't run Exchange 5.5 on a GC if that GC needs to talk to other GCs in your Win2K site. Therefore, Microsoft supports Exchange 5.5 on a Win2K GC only if you have only one Win2K GC. In that case, the Microsoft article "XADM: How to Configure Exchange Server 5.5 to Run on a Domain Controller or Global Catalog" (http://support.microsoft.com/support/kb/articles/q275/1/27.asp) explains how to change the Win2K configuration to make this work (and how to change it back when you want to install Exchange 2000 on that computer). In addition to the NSPI port, you might also have problems with the Lightweight Directory Access Protocol (LDAP) service on Exchange 5.5 unless you change it to use a different port. The Microsoft article "XADM: How to Change LDAP Port Assignments in Exchange Server" (http://support.microsoft.com/support/kb/articles/q224/4/47.asp) gives details for that procedure.