B/c you don't have the right codecs: http://www.land.ee/d_nimo_codec.html OR else http://www.click-now.net/html/K-LiteCodecPack.htm

ALternative is to get a back up player which plays everything (xcept the odd wmv file) & has in-built codecs & is free: VLC: http://www.videolan.org/vlc/ (choose nearest mirror site)