How to reset the Administrator password on a Windows NT/XP/2000 computer
If you're able to reboot the computer and boot on a floppy disk or CDROM, that's quite easy (but that's easy on almost any operating system on a PC).
You just have to use the bootdisk or program avaible on this website :
http://home.eunet.no/~pnordahl/ntpasswd/
You boot on the floppy disk, then you'll be able to reset the password for any account.
An other solution if you don't want to reset the admin passwd, is to write down on a paper the password hash for the account. Then you can use this webpage to get the password from the hash in a few seconds :
http://lasecwww.epfl.ch/~oechslin/projects/ophcrack/
By the way, while you're on this webpage, you can read
this article (or
this one in french) about rainbow tables and how they used them to pre-calculate most windows passwords, it's quite interesting :)