Ok, yeah it is because you are using storage server, that can't become a domain controller. (not supported)
...same thing applies to 2008 but this FAQ is for 2003 R2
http://www.microsoft.com/windowsserversystem/wss2003/productinformation/FAQ/default.mspx
A dedicated file and print server is a server built on Windows Server 2003 as a fixed function device. All of the functionality unrelated to file serving has been removed, increasing reliability and lowering the overhead on the machine CPU.
Do you have another Windows 2008 server (that is not storage server) If you do that can be promoted.
Thanks
Mike