Make sure that the domain and forest are both in the correct mode. I think it’s called native mode, it might be Windows 2000 mode. They changed the names from Windows 2000 to Windows 2003, and it’s been a long time since I’ve looked at a Windows 2000 domain.
You can check the Forest Functional Level, to do this go to Active Directory Domains and Trust, right click the Domain and choose Raise Domain Functional Level, this will let you perform your task. More info can be found here.