The windows login screen may appear black if the registry entry responsible for the colors becomes corrupted. In this case it has values of ( 0 0 0)… or black…..
This is simpley fixed by making the appropriate changes to the registry entry. The simplest way would be to take a working reg entry from a different Windows Server 2003 system and add it to the system encountering the problem.
The following key is what is probably causing the issue