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 [...]