You cannot install a read-only domain controller at this time. You must first run "adprep /rodcprep" from a command window ...

You cannot install a read-only domain controller at this time. You must first run "adprep /rodcprep" from a command window on any computer in this forest. The Adprep utility is available on the Windows Server 2008 installation media in the \sources\adprep folder.