"jumbled, blocky, and fuzzy" is hard to really tell what you mean is wrong. In media player classic, go into the options somewhere and switch between overlay and VMR9 for the renderer (don't remember where those options are offhand, don't have mpc on this pc).
I used to have problems playing video files with overlay (on a 6600gt) and had to use vmr9 for everything. I switched to an x800 and the problem went away.
Don't know if thats your problem, but something worth trying at least.
Other problem might be the codec you are using. What type of video is it? Maybe check to see if there is a newer version of the codec and reinstall it.