How to add an existing non-Administrator to the Administrators group?

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http://superuser.com – A friend popped round their family laptop after discovering that his wife's user account fails to logon with the following error message (it's a Dell Inspiron N5050 running Windows 7 Home Premium Edition): The User Profile Service failed the logon His wife's user account is the one created when Windows was first setup during the initial Dell OEM first-time Out of Box Experience. This account is a Windows administrator and is the only administrator. There is another account used by their son which does not have administrator rights. I followed the steps that apply to this scenario descri (HowTos)