Boot into safe mode
Go to Start, then Run, and type in
and press Enter.
In the dialog that appears, you can either add an account or check the
properties of an account. Clicking on the Properties box will give you
another window. There will be a tab that says Group Membership. On that tab
you can click Other, and in the drop down box will be all sorts of account
choices - Administrator included.
You can also go through Control Panel...Administrative Tools...Computer
Management to achieve the same effect.