Are you sure that you added the new DC to the current domain? Seems like the new dc was promoted to a new domain and not an existing domain.
Did you prep your domain for a 2003 server? Does both of the DCs in AD Users and Computers have all the same data when it comes to users?
Here are the steps to add a Windows 2003 server DC.
http://www.experts-exchange.com/Software/Server_Software/File_Servers/Active_Directory/Q_23762383.html