Vista Aero Taskbar Thumbnails

I have a problem where a machine in our office running Vista keeps showing the taskbar thumbnail even if the user has switched to a different app. The thumbnail does not dissappear and instead launches a different thumbnail depending on which application the user has run the curser over on the taskbar.

I have checked the theme settings but cannot find a way to set this thumbnail to close after a certain period of time. Any ideas here?