You are using Windows Vista, correct?
You should be able to re-boot in safe mode and log in as the built-in
administrator. This account generally doesn't have a password and is only
available in safe mode.
From there, you can get to User Accounts and change passwords.
When your restart your computer, press F8 right before you see the Windows
splash screen. This will bring up the Advanced Boot Options menu, from which
you can select Safe Mode.
- JB
Vista Support FAQ
http://www.jimmah.com/vista/