Hi madshi,
first of all: Great work you are doing with madVR!
At least since the latest build 0.79 I am getting very often a black screen when starting a video in MPC-HC. I already tried to play around with the madVR options, but nothing changed the situation.
The sound of the video is audible, but I do not see the picture. If I press CTRL+ALT+DEL the video sometimes appears (sometimes have to press "Switch to" before), but sometimes MPC-HC closes immediately. I prepared a log, where I opened MPC-HC in fullscreen, the picture was black and MPC-HC closed after pressing CTRL+ALT+DEL.
http://www.mediafire.com/?a538o2rial1gxpl
Maybe this log helps and I would be glad if you could find the reason for this behaviour.