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.
i know that having video' files at bad resolutions will cause tearing in the clip, and maximizing can fix that issue, so maybe you have set your monitor to a bad resolution? This is something i have never experienced (bad resolution causing tearing), but i have heard about it. Give that a shot, switch the resolution and see what happans, wont take long, and cant hurt.