Enable the Administrator account on the Welcome Screen

Option 1:

• Download and install TweakUI from the Powertoys for Windows XP collection. In the Logon section, you can hide, or make visible, User accounts on the Welcome Screen.

Option 2:

• Click Start, Run and enter REGEDIT Go to:
  HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\SpecialAccounts\UserList (note that there is a space in the Windows NT). Right click in the right pane and select New, DWord value.
• Give the value the same name as the Username on the Welcome Screen (it must match) for the account you wish to hide/show. Double click the new value and set it to 1 to show the account on the Welcome Screen, or set it to 0 to hide the account.
  Example: Doug is a user on the computer, Doug would be the name you give the new value.

The individual Powertoys for Windows® XP are available here