You could try to use a different media player such as the free VLC player with built-in codecs which can play almost all media files without problems. You can download it from here:
http://www.videolan.org/vlc/
As a side note, to preview files with it you'll have to open the files directly from the Incomplete folder and not via the LW preview function.

Otherwise you could google for the K-Lite Codecs Package and install that. Though I personally recommend the VLC player, since I think the WMP is crap....

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