I have this same problem, i.e. a slightly shifted ghosted image of everything, when I use my Xbox 360 with my PC monitor via a third-party VGA cable. The cable is probably not the greatest, but I loaned it to my friend and he reported that it worked fine (i.e. he did not notice any ghosting) with his Xbox 360 and his monitor. I don't have the problem when using component to my regular HDTV.
As noted above, it could be the cable, or it could be interference of some type.