im presuming the video thumbnails are not avi or mkv as they were already showing, i would hold off installing a codec pak until you have tried a few things first.
(after all windows should display video thumbnails by default.)
check you have thumbnails enabled, in an open window click organise then select folder and search options/ click the view tab then make sure the "always show icons ,never thumbnails is unchecked.
if it is unchecked then
try rebuilding the thumbnail cache... Thumbnail Cache - Clear and Reset