Before you can introduce Windows Server 2008 domain controllers into existing Windows 2000 or Windows Server 2003 domains, you must prepare the forest and domains with the ADPREP utility.
ADPREP /forestprep must be run on the Schema Master (to find it type netdom /query fsmo) of a forest and under the credentials of someone in the Schema Admins and Enterprise Admins groups.
ADPREP /domainprep must be run on the Infrastructure Master of a domain and under the credentials of someone in the Domain Admins group.
In order to run ADPREP, insert the DVD media of Windows Server 2008 into the DVD drive of the appropriate Windows 2000/2003 DC, which, as noted above, should be the Schema Master of a forest.
Browse to the support\adprep directory.
Open a Command Prompt window (Click Start > Run > CMD > Enter), and drag the ADPREP.exe or ADPREP32.exe file to the Command Prompt window.
Type: adprep /forestprep
Make sure you let the existing Domain Controllers replicate all the changes throughout the entire forest BEFORE proceeding to the next step.
Next, go to the Infrastructure Master of each domain that you wish to upgrade and insert the DVD media of Windows Server 2008 into the DVD drive. Repeat the instructions to open the Command Prompt window, and type: adprep /domainprep
For more information see [http://www.petri.co.il/windows-server-2008-adprep.htm Petri Knowledgebase]