Try going to Control Panel (Control Panel Home, not classic view) User
Accounts and Family Safety, User Accounts, Manage Another Account; and
see how many users there really are. If you are the only user as you say
you are, then: Go back 1 page, and click (above Manage Another Account)
Change Your Account Type, click Administrator, and click Ok. When that
is done, log off and back on. Then try again.
This is a shot in the dark because you may need a password to change
account type. But it may work. Try it.