By default, the taskbar will show
thumbnail previews of open windows and programs when you hover the mouse pointer over it's taskbar icon while using a
Aero theme.
When you are using a
Basic or High Contrast theme, the taskbar will only show a
list of tabs (list mode) for the open windows and programs (ex: Internet Explorer) when you hover the mouse pointer over it's taskbar icon, and never any thumbnail previews.
While still using a
Aero theme, this will show you how to change the number of
thumbnail previews that will display for a icon on the
taskbar before it switches to
list mode instead in
Windows 7.