Many folks, including myself, have complained about the Monitor window brightness while editing video. I have found a solution that seems to work for me. After playing around with several utilities that allow adjustment of the video monitor gamma I settled upon using one named 'Gamma Panel'.
http://www.majorgeeks.com/Gamma_Panel_d2796.html
I have a hot key sequence set to adjust my monitor display gamma to 1.5 and another one set to restore the default. I set my PrEl v2 'User Interface Brightness' in Preferences all the way left to Darker. I also de-selected 'Use Windows Background Color' since this arrangement seems to give the best contrast to all the elements.
I don't normally like to run any application with such a dark appearing interface but this does make the Monitor window display look good to me. It also makes viewing videos in VirtualDub watchable for the first time.