If it is running OS X then you can insert the OS CD, boot from it and add a user from there.
We had a HDD fail on one of our office Mac laptops (a power pc G4). Apple repaired it and I restored all files etc from a backup. I was the administrator during the restore. Before I could get back, the user changed that account to a standard account and changed the name. Now there is no way to change the account back, because there is no admin to authenticate the change. Anything I can do? I am not an advanced computer/Mac person. Thanks