How To: Activate Hidden Windows 7 Administrator Account

Using Computer Management Policy

Another way of activating the administrator account in Windows 7 is via Computer Management Policy.
1. Type compmgmt.msc in the search bar and hit enter.
2. After the Computer Management window pops up, navigate to Local Users and Groups -> Users.
3. In right-side pane double-click on “Administrator” account. It’ll open its Properties.
Deselect “Account is disabled” option and Apply it.

Stolen from : http://www.tomstricks.com/how-to-activate-hidden-windows-7-administrator-account/