Recently (in the past few days,) perhaps following the latest Windows Update, it appears the tick box to allow automatic logon in "control userpasswords2" or "netplwiz" has disappeared. I have two machines running Windows 8 and the option is no longer available for either.
Both machines user a Microsoft account, rather than a local account, for the primary user to logon.
The only other recent change I can think of, and I am confident this change was made well before this issue arose, was enabling HomeGroup.
How can I re-enable automatic logon for the affected user?