What type of previes do you mean? Videos or audio files? See means something I guess lol Make sure your video program is the default for the type of video file you're trying to preview. eg: Setting Default video app

BTW for previewing I'd recommend VLC - (click on link) _ (choose nearest mirror site) as it has in-built codecs & is generally very reliable. To see what it can do see http://www.videolan.org/vlc/features.html which includes audio formats, video & audio streaming, cd & dvd & vcd playback, etc. You can also downld extra skins for it.