HOW TO UNLOCK YOUR COMPUTER WHEN YOU FORGOT PASSWORD

Unlock your computer without the aid of a password-reset disk by accessing the default Administrator account from Safe Mode

Restart the computer.

Press and hold down the "F8" key as the computer is booting. Release "F8" when a list of options appears and select "Safe Mode With Networking" from the list.

Click on the "Administrator" account.
Press the Start button on the lower left-hand side of the screen.
Type "control userpasswords2" in the text field that appears and press "Enter."
Uncheck "Users must enter a username and password" and restart the computer. 
It will be unlocked. 

Source: http://www.ehow.com