thumbnails unavailable for all movie files

i have windows media player 6.4 set to play avi and mpeg
files and windows media player 9 set to play all other
movie files.

one day i accidentally set windows media player 9 to play
mpegs (or probably avi files) so I try to set it back,
which was a little tricky but somehow, the end result was
how i wanted things set BUT now my movie files no longer
show thumbnail icons in windows explorer. Windows XP just
shows giant icons for the movies. How do I get the
thumbnail images of the movies to show up again? I've
tried unsetting windows media player 6.4 and 9.0 from
playing any movies and reseting it back but I still don't
see thumbnails. I've even tried changing the view to icon,
tiles, and back to Thumbnails but nothing changes. What
can i do?