The Aero function may well have become disabled - perhaps you decided at some point to switch to Classic View?
Try right-clicking on the desktop, select Personalize, select one of the Aero themes available.
If this doesn't do the trick, and making sure you have selected an Aero theme:-
You may have disabled the taskbar thumbnail previews option.
To enable it, Right click on computer and click properties.
Click on "Advanced System Settings" and go to "Advanced" Tab and click "Settings" under "Performance"
On the Visual Effects tab you can select "Adjust for best appearance". Click apply.
Log off and login again.
Hope this works.