You might need codecs for your WMP (try the XP codec pack in the link below). Or if you like, you could try VLC player. It has codecs installed & plays most things! There's a link for download below, take the nearest morror site
Codecs & VLC

*The only difference with VLC is that you'll have to launch by opening your saved folder outside of LW. Go to your library, click on Saved Files & explore... your saved folder opens for you! Launch from here. Same deal for previewing, except you'll have to open your Incomplete folder.