How do I log back into administrator account

I have a joint administrator account. I read Microsoft advice about not being an administrator AND user for security reasons so I kept my status as an administrator and made myself a regular user, my name only, on my HP PC with the Vista operating system. Now I cannot even access my administrator account, which I know the name to and if protected by a password, know that as well. How do I get back to my administrator's account? I have been to Control and selected User Accounts but am blocked by "group policy" from doing anything.