Win2000 pro on a NT 4.0 network

One of our dept. keeps adding to his computers with out checking with us first! The computer is 2000 hardware ok. I need to add it back into the network. I went through the network neighborhood and added the IP, network id, default id. Through my computer I tried to change the name, it was the the wrong name by the installer. It said the name was already in use, so I removed the computer from the server. And tried a second time, now it says invalid user id. I logged in as admin I should have full rights to everything.