Show Window Previews enabled, but not working

As the title suggests, Window Previews have been enabled, however I do
not get a small thumbnail when hovering over windows in my taskbar.

It used to work when I first installed Vista. I can't really put my
finger on when it started happening, but it was quite recently.

I have tried Googling this problem, but I have only found forum posts
where the tick box is greyed out. My problem is that I have it ENABLED,
but it doesn't work.

'[image: http://i32.tinypic.com/2m62vb6.jpg]'
(http://i32.tinypic.com/2m62vb6.jpg)

My graphics card is a 9800GX2, so it is Aero capable.

Does anyone know how to fix this?

Thanks.


--
Ramzy