How the hell do I pixelate something in a video?

I want to know how to pixelate/blur particular objects in video clips, e.g license plates. Windows Movie Maker, of course, doesn’t have that ability, as far as I know. Every program that seems to have this ability, though, is a trial or some other form of paid program “Available for FREE!”, like Camtasia Studio, which had an awesome tool just for this, but the trial expired after a month. Any ideas that don’t require terribly complex video-editing knowledge or purchased programs?