Hi and welcome to the Forum. This should solve your problem:
Reset User Account Password - Vista Forums
EDIT
Sorry this is no good to you as you can't log in at all. My mistake

I think but I am not sure that this will mean investing in some third party software. I have found a couple on the web but know nothing about them at all so perhaps a member with more experience will be able to help. Sorry to have confused the issue but Welcome anyway