Might be a 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 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.
If this player can't play the file, I'd recommend checking your system for possible intruders using programs of the following list:
http://www.lavasoftusa.com/software/adaware/(free) http://www.grisoft.com(good free AVG) http://www.ewido.com(>>now called AVG Anti Spyware<< free 30 day trial -the best in my opinion) http://www.ccleaner.com/download/(free) Spybot - Search & Destroy (FREE) 