Duplicate computer name prob

I have accidently called a new computer(win2kpro) the same name on the network as an old PC (win2kpro) (name:room4) I managed to rename the new pc to r4 (eventually) But now when i try to log on to the old PC i get this – error message: The system cannot log you on to this domain because the systems computer account in its primary domain is missing or the password on that account is incorrect. The PDC is win NT. Also I cannot log on to the machine as a stand alone because the user changed theadmin password and cannot remember it! On the server: event id 5722.

Please help