http://www.sevenforums.com/tutorials/507-built-administrator-account-enable-disable.html
Through a Elevated Command Prompt
NOTE: This option can be used in all editions of Windows 7.
1. Open a elevated command prompt.
2. To Enable the Hidden Built-In Administrator Account -
A) In the elevated command prompt, type
net user administrator /active:yes and press Enter. (See screenshot below)
B) Go to step 4.
3. To Disable the Hidden Built-In Administrator Account -
A) In the elevated command prompt, type
net user administrator /active:no and press Enter. (See screenshot below)
4. Close the elevated command prompt.
5. Log off, and you will now see the built-in Administrator account log on icon added (enabled) or removed (disabled) from the log on screen. (See screeenshot below)
NOTE: Click on the Administrator icon to log on to the built-in Administrator account.
