Adding a server in a W-2000 LAN

Hi There, I have a LAN controlled by a windows 2000 server that host the active directory. At the same time have a small group of users (4) and another sever (W-2000) in the same LAN that works in a work_group environment. I tried to bring the work_group to the domain, but everytime that I trien do make the work_group server join the domain, I have this error message: “Error when trying to join “domainname”, the current credentials are in conflict with another group of existing credentials”
What should I do?…