You do not need to reinstall if you forgot the admin password.

There is a boot disk you can download at:
http://home.eunet.no/~pnordahl/ntpasswd/

that will let you reset the password of any account on NT4, Windows2000 or XP.

Just follow all the default settings which are for the admin account. Used it many
times with great success.

They also have a bootable CD image you can download.