I tried 178.24 (uninstall, driver sweeper in safe mode, installation of 178.24), but it's still the same :-(
One more thing I noticed (with every driver version so far):
In PowerDVD I can check the hardware acceleration (Nvidia PureVideo) box in the config dialog and it stays checked if I restart PowerDVD.
But as soon as I drag a HD video in and it starts playing, the box is no longer checked.
In PowerDVD 7 the box grays out then. In PowerDVD 8 it doesn't, but checking it isn't persistent and doesn't change anything.