I ran into some cases, where the Windows 8 CP start screen didn’t respond any more. When I clicked on a title, the tile would show the “button click transformation”, but would not start any app. It would not even show the desktop or by pressing CTRL + SHIFT + ESC show the task manager. The only thing that worked for me, without rebooting the system, was sign out and sign in again. The virtual machine that I was running would still be running after the sign in, sign out action.