If it isn't a virus then it 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 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.
Maybe checking the file using G-Spot shows you the true nature of the file. You can download it here for free:
http://www.free-codecs.com/download/GSpot.htm
And if it turns out to be a bogus file....
Check your system using any of these programs (I reckon using all of them):
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)
If you can't get rid of it go here and get some free computer help:
http://norbiesworld.co.uk/computerhelp 