You receive one or more of the following:

The operation failed because:

A Schema validation check failed.

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

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

The operation failed because: The Active Directory installation requires domain configuration changes. Run the ADPREP /domainprep on the domain controller (null) to make these changes and then proceed with the Active Directory installation.
The server is unwilling to process the request

The Windows 2000 Active Directory forest and domain need to be prepared…..

Microsoft Knowledge Base Article 278875 describes these issues: