Blocking in my Videos

While watching some high definition videos from MC-Nudes I had downloaded a while ago, I noticed that some of them presented considerable blocking artifacts, independently on the video player I used, so I updated to the last versions of K-Lite Codec Pack and DivX Player, but the artifacts remained the same, although DivX Player managed to avoid sporadic frame freezing now presented by the other players using the latest codecs just installed. I spent some time tweaking options in several players with the hope to get rid of those artifacts but the best result was the aformentioned DivX Player advantage.

Today, among the videos I had left to watch, I found a copy of one clip I had already watched earlier. This copy, unlike the earlier one, presented blocking artifacts galore (click on frame capture above for a larger view), so I set to make a first quick comparison of both files: they have extactly the same file size and video dimensions. So I used GSpot to export detailed lists of their properties and found that the blocky file reported a slightly slower bitrate and surprisingly almost half the frame count. I don't know much about video codec internals so I can't explain how both files can have the same file size while presenting those key differences. The question is what has caused the blocky version? Did MC-Nudes actually release both files using different encoding parameters or something or someone in between had hacked or corrupted the video? Is the blocky version actually hiding some trojan horse-like surprise?

For now, I keep the good version of this one, but I still have other blocky videos I would like to know how to fix, if this is possible.