Surexh,

 When trying to play online video, if you get that message ("you need some coded to play this video" or something similar..), dont think it is necessarily the truth. The site might be fooling you..  There are many online malicious sites, they look like video sites, but in fact they download virus / Trojan horse or something else, if you honestly download what they ask you to download.

Next, if it is video on your computer.. and you can not play, try VLC media player (free stuffs, google around). It should play most of video encoding format.