Two Options:
# Download Hiren Boot CD, burn it to a CD and boot from it and follow on screen prompts.
# If you're lucky and can get into a guest account and havent downloaded the update that patches this command line action, then you can type this into a command prompt without the quotes:
[code]"net user <username> <new-pass>"[/code]
Where <username> is the username of the account needing the change and <new-pass> is the new password
For Example,
[code]net user jerry abc123[/code]