First, you need to download an Ubuntu livecd iso file. Write the iso file to a cd or usb drive. In any case, make sure the target system can boot from the medium you are writing your iso file to.
If you use Ubuntu, Ubuntu will mount the Windows drive for you. Ubuntu will add the windows drive to the left pane. If it doesn't, you might try to manually mount the system. You need ntfs-3g for this.
Open up a terminal:
mkdir /mnt/windows ntfs-3g /dev/sda1 /mnt/windows –o force cd /mnt/windows/Windows/System32/config
This assumes /dev/sda1 is the Windows drive. As mentioned, you shouldn't be doing this as Ubuntu is smart enough to mount the system
From the graphical interface, you will need to enable the multiverse to find chpwnt. Go to the system settings.
Choose Software & Updates
Enable the community-maintained free and opensource software. Close and choose to reload when prompted.
Now start a terminal to install chntpw or do this via the graphical installer. Go to the top left, click it, and type 'terminal'. Click on terminal.
sudo apt-get install chntpw
After installing chntpw, from the terminal, navigate to the mounted windows drive.
The hive (sam file) might be SAM or sam, find out:
Select a user id from that list:
From the resulting menu, choose 1 to delete the password, and 2 to unlock the account. Exit with q, and save the hive.
Reboot the system into windows.