Since last week, Win7 requires user logon instead of bootng desktop

Sometime last week, Windows 7 on my main PC changed its startup behaviour. Instead of booting up straight to the desktop as it alsways used to, it boots to the list-of-users screen, requiring me to click on the only account icon (there is only one 'admin' account, which has no password). Only then does it load the desktop.

I have unticked "Users must enter a name and password to use this computer" on the User Accounts screen, and on the resultant Automatically Log On screen, specified the only account, leaving the password field empty as this account doesn't have one. This does not work - Windows 7 still requires me to click on the account icon to load the desktop.

(I notice that on the User Accounts screen, a second user called 'HomeGroupUser$' is listed, but I assume that this is some system user, as I cannot select it or anything.)

Is there some other setting that might have got corrupted causing this behaviour?

Another addendum: I have also noticed that on booting up, after the Starting screen and before the login screen, there is about a minute of black screen with cursor. That never used to happen, either. Something very odd going on here.