I'm trying to edit my friend's videos which I have downloaded from Youtube.  When viewed with Windows media player they are fine, but when opened in the editor they are pixelating badly.  I have tried with another video of his off Youtube, filmed and uploaded in exactly the same way, and it is ok.

I've edited other videos with no problems, but they were downloaded from my camera.

Windows 7 Ultimate

Magix Movie Edit Pro 2015 Plus

Plesae help!!!