The built-in Administrator account has a higher access level to the system than a normal local administrator account. By default, the built-in Administrator account is disabled. For information about enabling and logging in to the built-in Administrator account, contact your system administrator. You can also check this support document for Windows Vista on the Microsoft support website: http://support.microsoft.com/kb/926183/en-us?spid=11737&sid=73.
For Windows 8, see the following article on the Microsoft support website: