So I was watching a DivX version of snatch and the first few mins of it were all horrible colours and blocks, the usual compression corruption. I decided that it would be cool to use it on purpose in something. It's a lot of work to do it all by hand in an animation program, so the question is: is there a way to do it on purpose?

hm either some pro tools allow adding noise fx via filters, or you would open the file with a hexeditor and throw some garbage values in it, though likely the latter is too much a game of luck

err, compress it?
You couldn't really tell it what to do then, but play with the values a bit, and you'll probably get what you want.

After its compressed, import it into your favourite video-editor, and paste it into the original file (you could even try to select the best bits and overlay those only.