Missing Administrator Account - Here is the solution

Once you have created regular user accounts, the default Administrator account vanishes from the Welcome screen, which you see when the computer starts up. Press Ctrl-Alt-Delete twice at the Welcome screen to retrieve the standard logon dialog. You can log on as Administrator from here. To switch among accounts, just click the Log Off button on the Start menu. You'll then see the Log Off Windows dialog box. Click the Switch User button, and you'll be taken to the Welcome screen where you can select and log on to other accounts.