For quite some time, my computer will not play any movie files I convert or download via windows media player, winamp, wmp classic, or even my ati media player. VOB files work fine. DvD's also play normally.
I don't think I'm missing a specific plugin, because it's universaly faulty.
This is a Microsoft critical update problem - they've screwed up again.
You will either have to system restore to an earlier date (before the update) or go to add/remove programs, check the box at the top (show updates) and remove the WMP update. Let me know if that works for you.