Note that this will reset the keychain. When you log in for the first time afterwards choose create a new keychain.
Hold down command R when starting up and you will boot into the recovery partition on the computer.
From there you can go to the Utilities menu and then Terminal.
Type resetpassword all one word.
A window will open up allowing you to reset the password for any account on the computer. Depending on the OS you may need to click on the HD in the top of the window before you are presented with accounts to reset.
Reboot and you should be able to log in with the new password.