Codec problems! .. We normally recommend using VLC as it has in-built codecs & will play all video & audio formats you will find on gnutella network, if they are valid & not spammed corrupted files of course.

To make VLC your default video player, see Setting Default video app

Elsewise, see CODECs list for Movies/Audio/ Alternate players / Video help forums/ video-mp3 fixers

More about video information here: Movie download issues (click on blue link) (2nd last post has information on best video formats.)