The built-in administrator account (Administrator) of Windows Vista, 7, and 8 can be used instead of creating a custom administrator.
In an elevated prompt, issue the following command to activate Administrator:
net user administrator /active:yes
Likewise, the following command disables the account:
net user administrator /active:no
Opening an elevated prompt
To open an elevated prompt, open the start menu (in Vista and W7), type Command Prompt and open it with admin privileges by pressing Ctrl–Shift-Enter. In W8, you need to right-click the start menu button and select the respective entry.
-  Instructions with screenshots