These steps were written for Windows 8.1, but they work similarly for Windows XP through Windows 10.
The Local Administrator account is disabled by default. The settings made in WorkGroup mode are different one than those of Active Directory Domain. So, the procedure to enable built-in administrator account requires a different approach. This can also be useful for computers that will be joined to a domain.
An administrator account is the at the hierarchy to manage all the activities for a system. Since the local administrator account is disabled by default, so in order to enable it, we need user who is the part of default administrator group. He can easily enable built-in administrator account using the steps illustrated below:
In this way, the built-in administrator account is bought out into action. You should notify the administrator about the change you’ve made to the system. This procedure also works in Windows 10.
Hope you find the article useful! http://www.thewindowsclub.com/enable-local-administrator-account-windows-8-1-workgroup-mode
Note: You really shouldn’t use this account for anything other than Administration or troubleshooting. Using the Admin account technically breaks the Windows Security Model, increasing the risk of having the computer compromised by malicious software.
First you’ll need to open a command prompt in administrator mode by right-clicking and choosing “Run as administrator” (or use the Ctrl+Shift+Enter shortcut from the search box).
Note that this works the same in all versions of Windows. Just search for cmd and then right-click on the command prompt icon in the Start menu or Start screen.
If you are in Windows 8.x or 10 you can right-click on the Start button and choose to open a command prompt that way.
Now type the following command:
net user administrator /active:yes
You should see a message that the command completed successfully. Log out, and you’ll now see the Administrator account as a choice. (Note that this screenshot is from Vista, but this works on Windows 7 and Windows 8 and Windows 10)
You’ll note that there’s no password for this account, so if you want to leave it enabled you should change the password.
Make sure you are logged on as your regular user account, and then open an administrator mode command prompt as above. Type the following command:
net user administrator /active:no
The administrator account will now be disabled, and shouldn’t show up on the login screen anymore.