If there are no other accounts in the administrator group, and the machine is not part of a domain where the Domains Administrator account could be used to logon and change the local Administrator password (the domain's Administrator group is automatically made a member of the machines Administrator group when the machine joins the domain) then the only way is to reinstall NT into a new directory (not the same, as it will upgrade and see the old password) and it will let you enter a new Admin password. Also if you have an old ERD that you knew the password at time of making, you could use this and restore the SAM and security portions of the registry.
There is also a piece of software from http://www.winternals.com that can break into an NT system (LockSmith) that will change any password. The software is not free, and will cost around US$100. Their new product, ERD Professional can also change passwords and is available from the same site.
A similar piece of software is also available from http://www.mirider.com that allows you to boot off of a set of disks and change the Administrator password. You can also use l0pht Crack for free.
or use this disk: http://home.eunet.no/~pnordahl/ntpasswd/