On windows xp, try this:
Boot computer and press Ctrl+Alt+Delete twice when you See Windows welcome screen / login screen. It'll show classic login box. Now type "Administrator" (without quotes) in username field and leave password field blank, press Enter and you should be able to login Windows.
Now you can reset your account password from "Control Panel -> User Accounts".
If that doesn't or you use windows 7 or vista, try this:
1. Enter a computer that can link to Internet. Download Windows password recovery Tool and install the software on that computer.
2. Run the software and create windows password reset disk by follow the instructions.
3. Eject the created CD/DVD and insert it into the locked computer.
4. Reboot the locked computer (it's necessary for you to change your locked computer's BIOS setting to make it boot from CD drive) and then follow the instructions to reset your password.
5. Now login Windows and set a new password.