You have to be really carefull which codecs to use and dont use DXVA its broken. I use Shark007 codecs and select LAV with DXVA off. Obviously with DXVA off you need more CPU and 4 are needed minimum for 1080p I have found. Bluray is something I tried and could not get to work. WinDVD and PowerDVD will no work in a VM. I used AnyDVDHD to overcome HDCP but VLC could not play back blurays at a decent rate. Also DVD playback under a VM fails for me. I always get a copy protection error and even with AnyDVDHD video is jerky but VLC will play DVD's ok so I use this and dont watch blu rays on my VM HTPC. Mediaportal is going to support non copy protected blurays in its next release but you will need AntDVDHD or DVDFab running in the back ground to play copy protected material.
You must turn off hardware acceleration in flash to get rid of the green screen. In windows 7 you can right click on the flash box to get up the settings but in windows 8 you must go into internet explorer options and turn off hardware acceleration for all explorer functions. I cant use HDMI sound as its just to glitchy and to get the on board sound passed through and sounding acceptable I had to reserve some CPU, I have 2000mhz reserved and this cleared the sound up. I have a device to mix SPDIF and DVI into HDMI so I will use this when I get time but they are expensive new, its a shame modern GPU's dont have the SPDIF input like the old Nvidia ones have.