I had the same thing happen when I added the DesktopLivePreviewHoverTime dword. I rebooted and, for about 5 minutes, my alt tab window stayed on top. Then, went back to old behavior. I've looked everywhere for a solution to this very annoying problem and can't find anything. When loading Windows 7, I did an upgrade. A coworker did a full install and does not have this problem. Is this related to some settings that got inherited from Vista?