So far, none of the suggested solutions has worked for me, but I accidently found a way to fix the ALT-TAB behavior at least temporarily and without having to close sidebar gadgets.
I have to open some application in full screen mode (for example pressing F11 in Windows Explorer, F11 in Firefox, or switching to Full Screen Mode in Adobe Reader).
Then I have to switch a few times back and forth (with ALT-TAB) between the full screen application and the desktop. After that, ALT TAB works fine.
I have not tested it on other computers, but maybe it helps someone...