To overwrite a Windows 7 password, one can boot from a Ubuntu LiveCD and run chntpw.
Unfortunately, installing chntpw is not so easy:
$ sudo apt-get install chntpw E: Unable to locate package chntpw
First open the Ubuntu Software Center, in the Edit menu click Software Sources..., and in the window that appears enable universe as seen below:
sudo apt-get update.
You should now be able to install chntpw using
sudo apt-get install chntpw.
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