chntpw

This little program provides a way to view information and change user passwords in a Windows NT/2000 user database file. Old passwords need not be known since they are overwritten. In addition it also contains a simple registry editor (same size data writes) and an hex-editor which enables you to fiddle around with bits and bytes in the file as you wish.

Only use this tool against systems you own or are explicitly authorized to test — see the disclaimer.