Hi,
I solved the problem without this info - thought I'd explain what I did (very simple solution without the need to alter hardware acceleration).
I had, the day before altered the settings for the Intel Integrated Graphics display. Basically I was poking around the program supplied by Intel for the graphics display. I altered a few settings. Thought nothing of it. The next day, when i started a video in Win Media Player (the latest version in 2013), the screen was almost entirely green with just a small part of the video playing in the upper left corner.
Initially I cold not work out what was causing this - especially as it worked fine in GOM media Player. Looked on the net. Tried new codecs etc. No solution. And my hardware Acceleration could not be altered as the button was greyed out.
Then i remembered that I had altered the Intel program settings. Returning these settings to the default settings fixed the issue immediately.
I hope this helps others with this problem.
Cheers
Owlinflight