If you are lucky, and the Administrator account has not been pasworded, you may be abe to enter safe mode, access the admin account and from safe mode change the password on the account that you don't have a password account.
Failing that go to these 2 sites.
http://www.petri.co.il/forgot_administrator_password.htm#20
has a link to this and other sites:
http://home.eunet.no/~pnordahl/ntpasswd/
There is sufficnet info and downloadable software to enable you to reset the passwords and let you gain access to the machine.
Panzer
December 2006