logon password

i have this client who has forgotten his administrator logon password, he is running on winxp pro and he has no other user accounts even his guest account is not enabled, i have tried logging in safe mode, vga mode but to no avail. I have also encountered a similar problem in win2000 where the clients logon password had expired and i still dont know the way around it. Can anyone help b4 i go the reinsatallation route?