- Type CMD in the Start Search box.
- Open a command prompt in administrator mode by right-clicking and choosing “Run as administrator”
- In the CMD box, type: net user administrator /active:yes
- Log out.
- You'll now see an option to log in as Administrator
- When you're ready to disable the visible Administrator option on the login screen, type this into the CMD box: net user administrator /active:no
(Thanks to: http://www.howtogeek.com/howto/windows-vista/enable-the-hidden-administrator-account-on-windows-vista/ for the help in logging in as Administrator.)