One I've used for a while is called Offline NT Password & Registry Editor. It's a free program you can burn to a boot CD or USB memory key, and then use it to reset your password.
Offline NT Password & Registry Editor works with pretty much every edition of Windows out there, including XP, Vista, and Windows 7. To use it, boot your PC using the program on a CD or on a flash memory key. Just follow the simple instructions on the screen to reset the Administrator account, and then reboot and log in -- that's really all there is to it.
That, and be sure to reset the password to something you'll remember this time. Might I suggest making your own cypher chart for creating strong passwords?
Photo courtesy Flickr user e-magic