Blatantly stolen from http://www.askvg.com/how-to-enable-activate-hidden-administrator-account-in-windows-vista/, a short Description how to enable the hidden Administrator Accounts in Windows Vista and Windows 7:

  • Click on “Start button -> All Programs -> Accessories“. Right-click on “Command Prompt” and select “Run As Administrator“. If you are prompted to enter password, enter the password and continue. You can also open Command Prompt in Administrator mode by typing “cmd” in Startmenu Search box and press “Ctrl+Shift+Enter“.
  • Now provide following command:

net user administrator /active:yes

  • Thats it. The Administrator account will be enabled. If you want to disable it again, simply replace "yes" with "no" in above command.

I have not tried the other two methods mentioned at askvg.com because the one above did work for me and is scriptable if need be…