According to http://support.microsoft.com/kb/555910
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
Note: Please replace the
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