I have a similar problem so I figured I'd tack on here.
I only have thumbnails for files generated by the codecs added by shark, ie mp4, mov, flv etc
I'm not using any other media players except windows
Basically all the windows files will not generate thumbnails.
I've tried deleting the icon cache, re-installing shark, deleting the windows media player library
nothing has worked.
so anything else?
thumbnails are generated for pictures
*** fixed my prob, I used those registrys to default wmv, mov, mpeg, mpg and avi