Boot into Ubuntu desktop using a USB stick
download the .deb file from this page https://launchpad.net/ubuntu/karmic/i386/chntpw/0.99.5-0+nmu1
Click the deb file, it should open the software installer, then click the Install button.
Ctrl-Alt T - to open a terminal
cp SAM SAM.bak # backup SAM file
sudo chntpw -u Administrator SAM
choose option 4) to unlock the user
or 1) to blank the user password.
Save the hive file (ie. SAM)
run "sudo chntpw -u Administrator SAM" and it should show "*BLANK*" in the user locked/disabled box.
running "sudo chntpw SAM"
sudo reboot and your user should be able to login again.
Note: On the problem I had, all users were locked out permanently on a single wrong password. I used chntpw to unlock the accounts and reset the passwords, then had to go into Control Panel -> Administrative Tools -> Local Security Policies. Then go to Windows Settings -> Security Settings -> Account Policies and alter the Account Lock settings.