This is just for information as I haven't seen it mentioned before.

I have the desktop background set to change every 10 minutes and to pick an image from one of several themes I have installed.

What I notice is that no matter what time I start the PC, the slideshow changes always auto sync to the clock (and to the millisecond) such that as the clock hits the hour, the image changes. Its quite clever the way it brings them into line.... because it thinks you haven't noticed... but I have