It could be a codec problem...So you can either download codecs for your computer or get the VLC player as it has them already pre-installed...Or get both....
If you are using XP this codec pac is good...
Download XP Codec Pack 2.0.7.1 - XP Codec Pack is one of the most complete codec pack
VLC media player (it's free)...
VLC media player - Overview
Also Bitzi is a great way to find out what the files are like before you download them...
How to use Bitzi Web Lookup .. & How to use Magnet Links & Direct Connect/Browse/Chat