Like in previous Windows versions, the built-in administrator account is disabled by default in Windows 8. You can easily enable it in Windows 8 with the command net user administrator /active:yes from a command prompt with admin privileges. However, if you are locked out of your computer, for example because you forgot your password, you need to enable the administrator account offline—that is, from a second Windows instance—to be able to log on with administrator rights.
Source : 4sysops
