when Win8 boots up to Start menu I position mouse cursor to lower-left corner. But the popup icon for switching to desktop does not show anymore. I have to right-click and choose Desktop. I have Win8 about a week, and I'm not sure what I changed so that the popup icon does not show anymore... Any help would be appreciated
This would be normal when Windows 8 is first started up since nothing (including the desktop) has been run yet. The pop-up is part of the Switcher that let's you switch between opened apps. Since the Desktop is considered an app, it doesn't show up in the Switcher until after the Desktop has been opened for the first time.
If you like, I find using the method in the tutorial below helps to make it easier to get to the desktop.
