Preparing an Existing Domain for a Windows Server 2008 Domain Controller
Many people who will be implementing Windows Server 2008 for the first time will be doing so in an existing Windows Server 2003 network. If you are going to be installing this Server 2008 box as a domain controller, you will first have to prep the forest and domain for the introduction of Server 2008. This is done with the adprep tool, found on the Server 2008 CD in the \sources\adprep folder. You will need to run both the adprep /forestprep and adprep /domain prep commands. These commands will need to be run on the domain controller on your network that contains the scheme operations master FSMO Role as they will extend the schema to support the 2008 server. In addition to this, if you will be installing a read-only domain controller (RODC), you will need to run the adprep /rodcprep command.