DISABLING BUILT-IN ACCOUNT

Hi all,

I jsut started working for a company that is using windows 2000 server. For security reasons my boss want me to disable the account that belongs to the guy who was doing my task before (they fired him). I try to disable that account but the system says that i cant be done because it is not posible to disable built-in accounts, does anyone know what can i do to disable the account of the admin who install the software on the server? Of course i cant delete it neither.