Administrator account is disabled by default on Windows 7.
Group Policy can be used to enable it.
Create a new GPO and go to: Computer configuration -> Policies -> Windows Settings -> Security Settings -> Security Options. On Right Pane click on Accounts: Administrator account status and change the settings to Enabled.
To enter a password for the Local Administrator Account, Group Policy Preferences can be used:
Go to Preferences -> Control Panel Settings -> and right click on Local Users and Groups. Choose: New -> Local User. From Drop-down menu on User name choose: Administrator (built-in).
Put the password and confirm it.