what is firevault?
I think you can use the original install disk. But that is only if you don't have FireVault activated.
This might help: Mac OS X: Changing or resetting an account password
^^^ worked like a charm!
I made a bootable rescue USB thumbdrive (per MacLife's How To Handbook magazine Spring 2010) and I couldn't log in with it. Booted with the original OSX disk, clicked on the change password option, selected USB thumbdrive, changed the password and now I'm in like flint!!!