so on the windows 2003 box you've run dcpromo to set it up as a domain controller,
(or used the server roles screen to choose it as a domain controller).
you have all the necessary instally, to make it a domain controller and it's up and working as a domain controller.
now what you need to do is go to the w2k box.
right click on your "my computer" icon, select properties.
go to the computer name tab.
and in the bit where it says "is a member of" work group.
click domain and add the domain name that you added on the 2k3 server.
press OK,
enter the domain admin username and password to add the box to the domain.
restart.
now the w2k box is on the domain and you should be able to log onto it using a username from the domain.