To prepare a windows 2003 domain for windows 08 Domain controller we need to run adprep
Insert windows 2008 CD and go to the following path:
D:\sources\adprep\adprep.exe /forestprep (or copy this folder from the windows 2008 CD and copy it to the C drive)
Run the following commands:
adprep /forestprep
adprep /domainprep
adprep /rodcprep