When you have the Aero theme enabled in Windows 7, you see thumbnail previews of the opened windows when hovering over the icons in the taskbar. However, there is a slight delay of 400 milliseconds for these thumbnails to load. If you notice the delay, you can decrease the delay time with the following Registry entry:
Hive: HKEY_CURRENT_USER
Key: Control Panel\Mouse
Value Name: MouseHoverTime
String Value: Default = 400 (milliseconds)
