Happens to me too. In my case, it has to do with some versions of the xvid codec. If you encode your own videos with certain versions of the xvid codec then Media Center has some issues when attempting to draw the thumbnails.
In some cases, Windows Explorer will actually crash because of the same problem....
if you do START->RUN and type REGSVR32 SHMEDIA.DLL and then browse to your folder with the videos (try thumbnail or details view) it might even crash there.
As far as my research can tell, it's a windows thing, and not a Media Center thing.
The fix? Remove any references to the xvid codec via uninstalling the codec. Install the new codec and reinstall Media Center.
JRiver folks: Do you think it would be possible to fix this somehow? MC 10 does not have this problem. IT should be noted that the videos play fine. At least an option to not scan for thumnails? I've looked all over the options and have not seen anything so I assume it's not there.