Sounds like you are missing the necessary codec. A codec is like a translation dictionary that tells your computer how to read a video or audio file. different media files require different codecs to be able to play them. for example there is a codec that allows media players to play mp3 files, another for wma and so on. videos require both an audio and a video codec. to make it more complicated, some many codecs only work for one media player. it may work for windows media player, but not for winamp or another player. Most codecs can be obtained free from various websites. search with google. some codec sites want you to pay... DO NOT PAY. Go to another site, or search P2P. look for 'codec pack' as this will provide many codecs all from a single install. be careful. media codecs have versions and are constantly being improved and changing. Old codecs might not play files that used the newer codec version when they were created.

There is a very good program called AVIcodec that will scan a media file and tell you what codecs the file requres and whether or not your system has them. It will also tell you where to get them, but those locations are the ones where you pay for them. Most of the codecs you would pay for are freely available through other LEGITIMITE sites that specialize in providing up to date audio and video codecs.