ok, i "solved" the problem.
although i'm pretty sure most ppl won't like the "resolution".
i changed the graphic adapter. not because of this problem, it was quite some time for an upgrade. i bought a Radeon X850 XT PE with which i replaced the 9800Pro. this apparently fixed the problem, every movie is displaying now.
a quite expensive solution tho.
however, since i tried before with all driver version from catalyst 5.1 to 5.6, i'd assume that the problem lies with the Radeon 9800Pro chip.
or maybe the Radeon 9800Pro/basic catalyst functionality combo.
grrrrrrrrrrr.