I have 3 domain controllers (2 which used to be Win2k, and now upgraded to
Win2k3; and 1 Win2k3). The domain and forest is operating at the Windows
Server 2003 level.

When I try to dcpromo my fourth Win2k3 R2 server, I get the following error:

The operating failed because:

The active directory installation wizard cannot continue because the forest
is not prepared for installing Windows Server 2003. Use the Adprep
command-line tool to prepare both the forest and the domain. Fore more
information about using the Adprep, see Active Directory Help.

"The version of the Active Directory schema of the source forest is not
compatible with the version of Active Directory on this computer."

If I go to the only two other domain controllers and try to do adprep
/forestprep or adprep /domainprep... it tells me that it has already been
prepared.

When I added the third Win2k3 server, it did not give me any problems at all.

Sounds like you are trying to promote a Server 2003 R2 system into a Server
2003 domain.  See this article for help:
-- http://www.msresource.net/content/view/60/47/

Thank you very much Paul, I just found your article when you replied to this.

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