When you press Prepare for upgrades in a domain containing Windows 2000 domain controllers (at Start / Help and Support / Getting Started / Installing and Upgrading the Operating System /How To), step 4 reads:

Disconnect the schema master from the network and do not reestablish the connection until step 9 in this procedure.

This is incorrect. DO NOT disconnect the schema operation master when you run the Adprep /forestprep command.

NOTE: See How do I upgrade Windows 2000 domain controllers to Windows Server 2003 and how to add new Windows Server 2003 domain controllers to Windows 2000 domains?