2 EXISTING DOMAINS, 1 SERVER

Beginners question here: After a migration to a new computer (windows sbs 2003), I have removed the original server, used the same domain name for the new server. There is an existing domain or workgroup that is present that the client computers are being logged on to. I dont want them logging on to that because it belongs to the last server machine. How do I get rid of it and have the users log on to the machine that is there now? What did I do wrong?.