You've got broken codecs and you've got some odd 3rd party codecs that may be interfering with the ones you need. If you have Gspot(if not, GET IT!) run that on the files in question and it will tell you if the correct codec is installed and if not give you an idea which codec you do need. Avoid "all in one" packs like NIMO, Tsunami, Angelpotion etc as they contain codecs that were(in some cases) hacked together and contain odd bits of code that can(but not necessarily WILL) result in a/v problems that require entire clean O/S reinstalls.
Another question is"Will any other apps play the file(WinAmp. Irfanview, etc)? And just to repeat what you've doubtless already heard: Update the audio/video/chipset/sound device drivers.