In order to logon as an Administrator you need to perform the following actions. These will work for both WinVista and Win7.

  1. Enable Built-in Administrator Account
    First you'll need to open a command prompt in administrator mode by
    right-clicking and choosing "Run as administrator"
    (or use the Ctrl+Shift+Enter shortcut from the search box)

  2. Now type the following command:
        net user administrator /active:yes

    You should see a message that the command completed successfully.
    Log out, and you'll now see the Administrator account as a choice.

  3. Disable Built-in Administrator Account
    Make sure you are logged on as your regular user account, and then open
    an administrator mode command prompt as above. Type the following command:
        net user administrator /active:no