Hovering cursor

I don't know what I did, but now when I hover my cursor over an icon in the task bar I don't get the small thumbnail, I get a written description. How do I get the thumbnails back?

Re install the right video driver. This happen when you update the video card through win7 update. it gives you worng video driver. Or you roll back or you reinstal the original driver

Actually it happened when I changed themes. Windows Aero only works with Windows themes.