In the Windows operating system, you don't actually need to know the administrator password in order to remove your computer from a domain. You do, however, have to know the password of the user account on your computer that has the permission to join and unjoin domains. This can be a secondary account, but there's no way to perform this task without providing some kind of password.
Click "Start" and right-click on "Computer." Select "Properties" from the drop-down menu of options.
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Click "Advanced System Settings."
Click the "Computer Name" tab.
Click the "Change" button at the bottom of the "Computer Name" tab window. This will open the utility that joins your computer to either a workgroup or a domain. Your computer must be connected to one or the other at all times. Click "Workgroup" to begin the process of disconnecting your computer from the domain.
Type the username and password of an account on your computer that has permission to join and unjoin domains into the "Windows Security" box on screen. This doesn't have to be the administrator account, but it does have to be an account with the appropriate permissions. Click "OK" to remove your computer from its former domain.