2. Rebooting made no difference -- I turn my PC off every evening, so it gets a reboot every morning
3. This past week the problem seems to have resolved itself. This coincided with a sudden change in sleep mode. I no longer get the rolling Windows images, which at one time used to crash the system if I clicked too soon to get a login bar or if I happened to wake the machine when the images were scheduled to change.
I can only conclude that, as I suspected, the problem was related to sleep mode. Pity about the pretty pictures, but a lot less hassle.
1. I have automatic Windows updates