You may be missing some codecs. Load an AVI that doesn't play into something like AviCodec, GSpot, or similar to identify the codec required. Some free utilities here:
http://www.videohelp.com/tools/secti...eo-identifiers
But sometimes it's the fault of WMP. To verify, try a better software multimedia player like MediaPlayerClassic or VideoLan. Free, here:
http://www.videohelp.com/tools/sections/video-players
I urge you NOT to install some super mega codec pack. That's asking for trouble. You may get away with it but often enough it buggers a system to the point of having to re-install the OS.
Good luck.
