Default setting for the Administrator account in Windows 7 and Windows 8 are disable, to use this administrator account you have to set it to enable first.
In Windows 8.1 you will not see the user and group folder from Computer Management menu. So you can't activate administrator account from Computer Management menu.
The way to active administrator account in Windows 8.1 :
• Run Command Prompt as administrator
From Metro UI (if you are on Desktop, press Windows logo on the keyboard to move or toggle between Desktop and Metro UI (User interface), then Write cmd, it will automatically search for Command Prompt.
After you see the Command Prompt, right click on it and choose Run as administrator.
• On the Command Prompt window, at C:\Windows\system32\ write :
net user administrator /active:yes
• You will see The command completed successfully, means activation process is success.
This way is not only for administrator account, you can do for all accounts.
To disable Account, change yes to no at the command.
net user administrator /active:no