lost administrator account password

I recieved a call yesterday from someone who has forgotten there administrator account password. They can not install a printer without logging on as the administrator. They asked for my help. I have not been approached with this problem before and do not know how to recover a lost admin password. Is there a way to recover it or a tool that will work or will it be necessary to re-install windows? I have not seen the computer yet as I don’t know if I will be able to help. They are running either xp or 2000, don’t know which one yet. Any help would be appreciated. Thanks.