You might need codecs for the player. There's a link to codecs below (I've used the XP Codec Pack with WMP 9) & also to VLC player (free) which has codecs installed & plays most things.
If you decide to have a go with VLC, you'll need to open your incomplete folder outside of LW & preview from there. Go to LW library, click on Incompletes then click Explore...incomplete folder opens & you can preview from there. Same deal for launching a completed video, except that you'll need to click on Saved Files then Explore, to open your saved folder.
Codecs & VLC