Taskbar will no longer auto-hide

I had to replace my hard drive, which meant a clean reinstallation of Windows 7 from reinstallation disks. Which also meant endless updates and "update operations". Somewhere in there, I set my taskbar to auto-hide, and for a while, that worked fine. After the latest round of automatic updates, however, my task bar will no longer auto-hide. The auto-hide box is checked. If I uncheck it, apply that, then check it and apply again, the bar will very briefly hide, there is some sort of flash - as if some window or something is displayed for a small fraction of a second - and the taskbar pops back up and stays.

Any suggestions as to what is wrong and what setting, if any, I can change to fix this?

I've marked this as solved because, after I posted to this forum, the taskbar decided it had had it's fun, and now it's auto-hiding again. I don't know what that fifteen minute "vacation" was all about.