chntpw is a Linux utility to (re)set the password of any user that has a valid (local) account on your Windows NT / 2k / XP / Vista / Win7, by modifying the crypted password in the registry's SAM file. You do not need to know the old password to set a new one. It works offline i.e., you have to shutdown your computer and boot off a linux floppy disk or on the dual boot system.
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.
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source:http://linuxpoison.blogspot.com/2011/01/13578167757233.html