I have one Win2003 AD Forest with one AD Domain in native mode. The DCs are Win2003 SP1. After completing all the steps leading up to the actual Exchange 2003 server installation (including ForestPrep), the installation fails at the point of "Configuring registry entries for Microsoft Windows Active Directory schema update".
I attempted to re-install the ForestPrep (setup with the /forestprep switch) and failed again. The message was the same. The setup log doesn't provide any help (unless I'm missing something).