Looks like you need xtra
CODECs then; here's a choice of codecs brands you can get:
1.
http://www.free-codecs.com/ 2.
http://www.k-litecodecpack.com/
3.
http://www.land.ee/d_nimo_codec.html 4.
http://lists.gpick.com/pages/CoDecs.htm
A piece of advice is to get a back up player such as one with built-in codecs. DivX player is good. Also this one which is very popular: VLC:
http://www.videolan.org/vlc/ (choose nearest mirror site)
Also see
Movie download issues for xtra tips on downlding movies & also consider doing Bitzi Lookups:
How to use Bitzi Web Lookup ... to verify the video is ok.