ophcrack is the tool that you need.
Download the live cd .iso file here: http://ophcrack.sourceforge.net/download.php?type=livecd
burn to a blank cd using nero, power iso or whatever;
boot the computer that you want to crack from the CD; allow opcrack to load;
you will be presented with a desktop like environment with a window in the middle. ophcrack will run automatically, attempting to crack all passwords on the host computer.
once cracked, the passwords will be shown in the table in plain text. make a note of these password, reboot the computer, remove the ophcrack cd from the drive, and enter the password to log into windows when prompted.
Hope this helps.