What did you do? Did you complete downlding it & then launch it? Someone else had a similar problem & after using VLC instead to play it, their problem was gone. VLC has in-built codecs, get it here: http://www.videolan.org/vlc/ (choose nearest mirror site) Making VLC the default video player will enable you to preview & launch via LW.
BTW don't use your playlist to play videos, it's not intended for that purpose.