What we like to use is called the Offline NT Password & Registry Editor Bootdisk / CD. It's a free download from their site but it takes a little skill to use.
The download links are at the bottom of the page. We use the third option which is making a bootable CD. Just print out the instructions from the page and boot with the CD in the drive. If your computer won't boot to the CD first go here for instructions on how to change your boot order.
There is another product we have used in the past that is way easier to use and that's called Renee PassNow and we will be doing a write up on it soon so check back for that. For now you can check it out on their website.