You need to run the ADPREP command to populate your current Windows Server 2003 Active Directory environment with the schema and permissions to support the new features of Server 2008. You can get the ADPREP software on the Server 2008 source DVD at \sources\adprep folder.
You have several options at this point:
/ForestPrep – Use this on the server holding the Schema Operations Manager Role. This preps the entire forest.
/DomainPrep – Run this after /ForestPrep in the domain that you want to add a Windows Server 2008 Domain Controller. This must be on the machine with the Infrastructure Operations Master Role for the domain.
Class: 6421A Configuring and Troubleshooting a Windows Server 2008 Network Infrastructure