This is a weird one. I've heard people say that it's impossible to preview an avi but I've done it a few times. Try getting extra codecs for your player, or have a look at VLC player, it has built in codecs. The only prob with VLC... if you're using Windows, you'll have to open your incomplete folder outside of LW & preview from there. For some reason it will let Mac users preview within LW but not Win users

Codecs & VLC