Yesterday, a friend forgot his windows 7 password, so asked me for help.
It was the first time for me to try to remove a Windows password.
After searching,  I got some solutions, but none works, when I tried to Google “remove Windows password under Ubuntu”, I knew chntpw.
chntpw is a software utility for resetting or blanking local passwords used by Windows NT, 2000, XP, Vista, 7, 8 and 8.1. It does this by editing the SAM database where Windows stores password hashes.
After installing chntpw under Ubuntu(a bootable USB stick), no matter ran
sudo chntpw -u Administrator SAM
or
sudo chntpw -u ox1f4 SAM
only showed the same error,
Cannot find value: \SAM\Domains\Account\Users\FFFFFFFF\V
I noticed the same question asked online without answer, after research the usage of chntpw, I tried,
sudo chntpw -i SAM
and bingo!
btw, if you have no Ubuntu Live USB stick, I recommend you use Offline Windows Password & Registry Editor, Bootdisk / CD, because it is only about 18M, and has both  Bootable CD image and USB Installation Package.
P.S.I forgot taking screenshot yesterday, and I simulated it  today. for text version, please visit https://gist.github.com/witwall/8305f16f92a7754fbad3
Edit user data and passwords
Please enter user number (RID) or 0 to exit
Unlock and enable user account
Clear (blank) user password
Quit editing user, back to user select
Write changes to hive files(SAM)