For previewing vidoe files (including clips) LW usually uses your default meadia player and not the integrated player in LW.
Maybe your default player has an codecs issue......
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 downlaod 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.
