I've been using Vista for a few months and one of the features I enjoy is the thumbnail previews of windows on the taskbar. Unfortunately, recently they have stopped appearing and I'm not sure why. When I go to taskbar properties, the option to turn on "window previews" is grayed out as seen in the following screenshot:
http://xbb.xanga.com/2e2f447b17534237690539/b187893617.jpg
Any ideas?