How do I fix computer names in a domain running Windows server 2003 and Windows XP Pro

We had  a new workstation added to the domain a couple of weeks ago using the same computer name and user name as the one it replaced. I decided today that the old system was a better system than one of the other ones we were using so I thought I could simply replace that one with the one that was replaced by the new one. I know I would have to change the ip to the ip of the one it was replacing. 1st. I shut down the new computer knowing that this one would have the same ip as was still in the old computer. 2nd I got the old computer connected to the domain and changed the ip to the ip of the older computer it is going to replace. I than turned it off and than started up the new computer as it needed to get back on line. Than went and started up the computer I just got done changing the ip in. What happened than was it displayed a popup stating there is a like idendity on the network. Some how I got into the computer as administrator thinking all I would have to do is run the network setup and change the name of this computer to the name that was in the computer this one was replacing. It let me go throught the change but at the end errored out with something like computer already exists . So than I tryed to add a number behind the name but would not except the name. Anyway I thought I would just go back to the older computer again and call it good for today. Problem now is neither of these computer are now known by the domain. The users can both login to the domain using different workstations so the user is still know by the domain but their computers are not.
HOW do I fix this. The Domain controller is Windows server 2003 standard. The workstations are Winows XP pro