Adding a computer to a domain

This sounds like you are trying to use the same computer name as an existing one.
Each computer must have a unique name, if you had an old computer using this name, ensure this is cleared from the server network prior to reusing it, otherwise use adifferent name.
:-)

if else fail try this last result.
Do the joining of the workstation from the workstation itself. Log on as a n administrator of the domain and create a computer account from the workstation to the domain controller. In that case your 2kpro will already have its own computer account. then create a user account in the domain controller fro the user of the 2k station to use when he logs on.

The Credentials conflict message appears when you try to authenticate to a server with 2 different accounts. For example, lets say you are logged in as User1 and you have a drive mapped to \\ServA\share1. You then try to map another drive to \\ServA\share2 (same server) but you put User2 in the 'Connect As' box. This will cause the error you're seeing.

What does that have to do with joining a domain: Nothing. I have never seen this message when joining a computer to a domain. If an oldcomputer account already existed with the same name, it would be replaced (done that many times). What steps are you taking to join the computer to the domain (be specific)?

The only other thing I would suggest is going to a command prompt and typing "net use". This will display all current connections. Delete any connections to your PDC.