previewing
 

need someone to explain step by step how to previes partial file. to save getting wrong movie

Well, actually LW has a preview function (right-click on the file and choose preview) but that might sometimes not work.
Another way would be to download the VLC player (it's free, has built-in codecs and is one of the most stable and most recommended players) here:
http://www.videolan.org/vlc/

If you want to preview a file with it, just open the player, go to file (left upper corner), choose open file, locate the file (should be in your Incomplete folder) and play it..........;)

And please, do not double post here in the forums, it's against the forum rules....