What I would do in this case would be the following:
1) Disable all thumbnailing and delete all thumbnail cache
2) Put only one of the videos in a specific folder
3) Download, install, and run RegMon so you have it up and running at this point before you move on
4) Re-enable thumbnailing of videos/pictures
5) Open only the folder that has the one video you put in there in step 2
6) Watch as it thumbnails
7) Stop RegMon and look for the "query" registry strings to get an idea of which registry value(s) it is looking for in order to thumbnail videos
8) Once you figure out which value(s) it's querying you can adjust the registry accordingly to either delete those values or change them so when it queries it, it no longer finds the string that tells it to thumbnail the video
Or, you could just wait until someone else does that or comes up with a different idea.
