This happened to me, too. I was trying to take the whole Windows Security thing seriously, and managed to mistype my password the same way in the password and confirmation boxes. My solution then was to keep retyping my password as fast as I could, and I eventually hit on the right combination of sloppy keystrokes ;)
On a more serious note, the official word from Microsoft is:
...which boils down to 'make a password reset disk'. You can enter a new password, but you will 'lose access to its encrypted files, e‑mail messages that are encrypted, and stored passwords for websites or network resources.'
You could also try a third-party reset utility, such as the Offline NT Password and Registry editor: I have not tried it on Vista, but it worked fine on my XP box back in the day, and claims that it now supports Vista. Website: