You lose your Windows password(s) and cannot log in to your machine. If you have a rescue disk, you should be okay. If not, you might have to turn to Linux for help.

The method that I describe in this tutorial can work with a Windows machine that either dual boots with Linux or does not. If the machine in question does not dual boot, you will need to download a live edition of a Linux distribution (I prefer Ubuntu for the task) and burn that ISO image to a disk (you could also use a Linux distribution on a USB drive). Either way, you will need to boot in to Linux to recover your password.