Chntpw is a nifty little little tool that allows you to boot into a light linux environment and manipulate a Windows machines password and registry. I have used this tool many times when someone asks me to fix their computer, but fails to give me a password. When dealing with user accounts, it allows a few options. You can clear a password for a given account, set a password for a given account, promote a user (make user a admin) for a given account, and unlock + enable a given account. Below is the process for clearing a password for a user. Be sure to check out the link before proceeding. I am not responsible for anything that you do.
First you'll need to download the ISO. If you are making a boot floppy, DO NOT unzip. Simply place the .zip on the floppy and proceed. To make a bootable CD unzip and burn the ISO to a CD. ISO Recorder can help you burn the image to a disk.
MAKE SURE THE COMPUTER WAS SHUTDOWN PROPERLY. If it was not shutdown correctly, the below steps will fail. If the computer was not shutdown correctly, boot back up to windows and shutdown from the login screen. Once the machine has been shutdown properly you will want to boot using the CD you just made. Some systems are set to auto boot of the CD, and if that fails, boot from the hard disk. I used a Dell and it uses F12 for the boot menu.
Once booted you will be presented with a screen like below. In most cases, just hit enter. If you are having usb, monitor, hardware issues, you may need to boot using one of the optional boot parameters. It should start to boot. It will prompt you a few times to make sure paths are correct, just hit enter.
Choose the "Password reset [ sam system security ]" option by pressing "1". Then choose "Edit user data and passwords" by pressing "1". It will display a list of users on the machine. Choose the user that you want to edit, and choose and option to preform on the user. After the desired operation has been done to the user(s), type "!", then "q". It will ask you if you want to write files back to the harddisk, choose "y". If you recieve errors choose "y" in the next step and repeat the above steps again to edit the user. Otherwise choose "n" to quit. After you enter "n" hit Ctrl - Alt - Delete to reboot the computer.