[dogscoff]

I use jriver's media jukebox for all my movie/ music playing, and it never gives me any trouble. THere's a perfectly usable free version, it handles nearly all file formats thrown at it and it's very stable and user-friendly.

As for your mystery problem... stupid question, but have you tried going to view in windows explorer and switching off the preview mode? (ie switch it away from thumbnail)
Windows can use a lot of resorces as it tries to generate all those thumbnails, and if you try to run a video as well it probably will act up.