We're starting an Active Directory (AD) migration by deploying a new Windows Server 2003 forest. We'll eventually move to Exchange Server 2003, but for the time being we want to deploy a few Exchange 2000 Server systems. Can we perform an Exchange 2000 ForestPrep in a Windows 2003 forest?

Yes. Several differences exist between Windows 2003's and Windows 2000's respective AD implementations, which is why you have to take precautions when you place the first Windows 2003 domain controller (DC) in an existing Win2K forest. However, you can safely perform an Exchange 2000 ForestPrep in a Windows 2003 forest because ForestPrep simply registers those schema extensions and changes that Exchange 2000 requires. Of course, when you install Exchange 2003, you'll need to perform an Exchange 2003 ForestPrep because it has a different set of schema changes. However, Windows 2003 has much more efficient AD schema replication, which helps reduce the effect of the schema changes on the forest.