These instructions are written using my N-vidia graphics card. You may find slight differences if you have another make of graphics card.
Why would you need to do this. Sometimes the hardware acceleration can require more processor power than the pc can provide, so by turning it off you can accomplish tasks, such as watching dvds. If you are using dual monitors or a tv as a second monitor and want to watch video on the second monitor, you'll need to turn off acceleration. This is apparent if you can view your media player in the second monitor, but the video is just black with audio playing.
See here for a video showing how to turn off acceleration.
Rt Click on an empty part of the monitor screen
Generally the default settings is full. You can gradually turn the acceleration down one notch at a time
Test to see if it works, if not try moving the slider down another notch.