chntpw allows you to view some information and change user passwords 
in a Windows (NT/XP/Vista/win7) etc SAM userdatabase file. You do 
not need to know the old passwords. However, you need to get at the 
file some way or another yourself. In addition it contains a simple 
registry editor with full write support, and hex-editor which 
enables you to fiddle around with bits&bytes in the file as you 
wish yourself.

There are 3 main executables
- "ntreg" (the registry library)
- "chntpw" (the password reset / registry editor frontend)
- "reged" (registry editor /export tool)

Quickstart example, where /dev/sda1 (or whatever partition Windows
has been installed in) is mounted to /mnt/windows:
  chntpw -i /mnt/windows/Windows/System32/config/SAM
The previous line should open a shell-driven menu.
