If you're under XP/2000, then getting to a command prompt and typing:

net localgroup administrators

will list all current administrators.

Typing:

net localgroup administrators %name% /del

Will remove %name% from the administrators group.

Now, if you need to reset the Administrator account's password, you can use either the user management tool, or you can use [link|http://home.eunet.no/~pnordahl/ntpasswd/|this] tool...