Try VLC (has in-built codecs): http://www.videolan.org/vlc/ (choose nearest mirror site) but if you want to preview from LW then you'll need to make it your default program for those particular video formats: Setting Default video app

I'm not sure about the x-video. It could be XviD (http://encyclopedia.thefreedictionary.com/Xvid+codec) which can be obtained here: http://www.xvidmovies.com/codec/ but I don't know it that's the format that the extension suggests. I think online video stores might offer that format.