To enable the build-in Administrator account, follow these steps:
1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.
4. At the command prompt, type net user administrator /active:yes, and then press ENTER.
5. Type net user administrator <Password>, and then press ENTER.

Note: Please replace the <Password> tag with your password for your administrator account, removing the brackets.
6. Type exit, and then press ENTER.
7. Log off the current user account.

To revert do exactly the same thing but change
net user administrator /active:yes to
net user administrator /active:no