I've had trouble getting any decent quality playback of 5DII .mov files on my PC. Found an open source player, mentioned on another forum.
VLC player is a free dowload and solved all my playback problems.
VLC is cross platform, it will run on Linux, PC or Mac, it's richly featured, even has the capability of being controlled via IP (even via an Iphone), has streaming capabilities and will play audio files.
VLC also has a conversion capability, how effective this is I have yet to determine, but I have successfully done a test (.25x reduction of a HD clip) mpg2 conversion, but the quality leaves a bit to be desired. Resolving the quality issue is hopefully a matter of adjusting conversion paramenters.
There is a setting that has to be changed in VLC player before Canons Video will play properly as follows.
Start VLC
Go to Tools > Preferences
In the lower left of the box click the checkbox “Show settings - All”
Then go to Input & Codecs > Other Codecs > FFmpeg and look for the option called “Skip the loop filter for H.264 decoding”
Change it from “none” to “all”
Restart VLC