You can see the crap here: http://i7.aijaa.com/b/00564/8749523.jpg
Ok, so that it looks like my GPU could be broken. But I get the issue with all video clips, no matter if it's from youtube or just play .wmv file. Even weirder is, windows media player while maximized doesn't have this issue. And those artefact boxes scale with the size of the clip.