Check that Windows 2000 Service Pack 4 installed on all the domain controllers and Exchange Servers. If it is not already installed install it now, after that run the Adprep.exe utility on the windows 2000 domain controllers currently holding the schema master and infrastructure master roles. The adprep /forestprep command must first be issued on the windows 2000 server holding schema master role in the forest root domain to prepare the existing schema to support windows 2003 active directory.
