ALT-Tab box displaying "hidden" programs in Windows 8
For some reason a registry value had been changed.
You will have to edit or create new one (if not exist):
\HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\AltTabSettings, DWORD
Set the value to 0 (1 gives it the "classic" look and feel), restart explorer.exe (or even Windows in some cases) and that's it!
Possible cause - Sysinternals Desktops v2.0