NT4 or NT5 (Win2000 and XP) but only tried it on my XP machine at work when we had to move it out of the domain but forgot to set the local admin password.......
There are several tools that can do this
but use at your own risk !
Boot disksanother site (Used this one myself)
And, really RTFM first. Also, changing the password of an user renders
encrypted files of that user inaccesable if there's no second account setup to read those encrypted files.
But so much for secure login.