I had this too ( ATI X1900XT )
I can confirm this is a media player codec issue and ATI WMV acceleration. Patchwork videos .. but not all ..
I first found a workaround with "ATI tray tools" :
In "tweaks" section / standart tweaks / System tab : Uncheck "WMV acceleration". This will correct the issue.
When i installed the game " X3 Reunion" and patch it, i saw that the pach installed a codec (?) named : " Windows Media format runtime", visible in the "add/remove programm list.
Since this game install, i can reactivate the WMV acceleration in "ATI tray tools" without suffering the patchwork videos.
So, you should make a search on this codec or whatever it is.
First time i saw this kind of crazy videos rendering, i thought that my card was Kaput

and i couldn't find an answer

. This was by great luck that i found those workaround above ...
Hope this help.
Link gave by Firebird should work.