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
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2 Answers

First open the Ubuntu Software Center, in the Edit menu click Software Sources..., and in the window that appears enable universe as seen below:

Ubuntu enable universe

Then run sudo apt-get update.

You should now be able to install chntpw using sudo apt-get install chntpw.

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Do this from the terminal

sudo add-apt-repository universe sudo apt-get update sudo apt-get install chntpw