Windows 7: How to enable live taskbar previews? They've gone, mysteriously

I am no longer getting the thumbnail previews on my taskbar. It may or may not be related that when I booted up this morning, I got a blue screen (I didn't watch the boot process - not sure when it occrured) and had to reset the PC.

I have tried the four solutions here to no avail:
Taskbar Thumbnail Previews - Enable or Disable

Except 1 because the homepage is down and the alternative source which I found is a .php file which I can only seem to open in gpedit or something and 2 because it doesn't apply.

Please, how do I enable my live thumbnail previews? I don't think I could have done it inadvertantly, because I haven't been toying with any settings.

Is your Aero on?

It is on. I was just closing some programs, getting ready to switch off my PC when suddenly my taskbar changed to its usual colour (I had changed the colour when choosing an Aero theme as per instructions in the linked thread above) and the previews came back. I reopened the two programs I knew I had closed (I'm wondering whether there was a third) but the problem didn't come back. I don't know how it came about.