I'm not sure about now but in a previous few versions of LW, when previewing VLC (& WMP) would not always open & play the file. In any case, to prevent preview files being created & taking up space, you can preview the incomplete file itself directly from the incomplete folder:
http://www.gnutellaforums.com/downlo...tml#post330410 (I need to update the sample image of the download window of LW 5.)