VLC - (click on link) _ (choose nearest mirror site) has in-built codecs & is generally very reliable for playback & previewing for LW. See its features here: http://www.videolan.org/vlc/features.html

The computer freezing might have resulted in the file not properly transfering to the downld folder. The act in itself might have resulted in the freeze. You might have received an error message but because you weren't present, you couldn't disable the error message whilst LW was at the beginning of the process. By error message I mean such as the file didn't verify properly.