Once you have run a service pack on a server and you try to add a server role, often it will not accept your original source disk. In this case you need to extract the service pack files. For example in the case of Windows 2003 Server SP2, use the command prompt with /x switch to extract the files:

WindowsServer2003-KB914961-SP2-x86-ENU.exe \x:c:\path-to-folder

Once it has extracted the files try to add the server role, this time hopefully with success.