Windows Built-in Administrator – Enable or Disable

From a Paid Windows Secrets article:
Restore accidentally deleted admin privileges
http://windowssecrets.com/newsletter/comcast-and-time-warner-cable-the-upshot-for-us/#story4

Here are the useful links:
http://www.eightforums.com/tutorials/9650-built-administrator-account-enable-disable-windows-8-a.html
http://answers.microsoft.com/en-us/windows/forum/windows_7-security/in-windows-7-my-administrator-account-has-somehow/03bfc548-9584-4d5c-b17f-b8bc71593b66

The commands you need are:
net user administrator /active:no
net user administrator /active:yes