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.
Do this from the terminal
sudo add-apt-repository universe sudo apt-get update sudo apt-get install chntpw