Windows

Do you mean you set up a 2000 Server with the wrong domain name during Active Directory setup, DCPROMO?
If this is the case, and you created your AD environment with the wrong domain name, you can't fix it.
You have to run DCPROMO again, remove the Domain Controller, then run DCPROMO again, and promote the machine back to being a DC.

If you have other DC as members of this same wrong domain, you have to do the same thing.
I am saying here that if you have a domain set up with the wrong domain name, you have to remove the domain ENTIRELY, then do it all again. Any user accounts in the domain will be lost. All domain-level security using these accounts will be lost.

Sorry, but you gotta do it all over again.