When I start D The only previews I see are for Deskscapes. The video previews (thumbnails) are blank when I switch to my videos which are mp4.
They showed up on my old hard drive. Since my old drive failed, I replaced it with a new hard drive and had to reinstall Win 7 Ultimate x64. Also had to reinstall all programs I had used on the old drive.
Thumbnails showed on my old drive. Small, large etc.
I was able to reproduce what you did , as you can see my previews work just fine ,
When you loaded your folder did you click No on sub folders .
Also try changing settings to Direct Show and Max Like my screenshots below
DO NOT CLICK ON DISABLE DESKSCAPES
IN SETTINGS MUST BE ENABLED.