Vista - No Alt-tab thumbnail previews

Hi all,
I don't really know how long ago this problem started I didn't really notice it at first but the previews for alt tabbing between programs along with the previews when hovering over the minimized program show up like the alt tabbing in Windows XP. Also my Windows key-tab does not work. Any ideas to what could be causing this? I checked my settings and they seem to be the same as when the previews where working. Is there some setting i could have accidental changed?
Any and all help is greatly appreciated and thank you all in advance.

You have disabled the Aero theme...

You need to right click on your Desktop, go to Personalize - Windows Color and appearance and select the Aero theme.


Note: If you don't see the Aero theme, it means your graphics card is unable to handle Aero theme. You probably have uninstalled the driver. Try updating your graphics driver and try again.

Try right-clicking an unused area of the Taskbar, then selecting Properties. Is the option "Show window previews (thumbnails)" turned on?

Thank you limneos that was it I must have accidentally changed it from the Aero theme to vista basic theme. thank you again

My Flip 2D (when I press Alt-Tab) previews have recently disappeared and nothing helps. Flip3D works but Alt-Tab is like in XP, without the previews. What to do ? Help.