After painting my computer's case with a cheap spray paint, it looked great. It was a nifty black machine of doom! Unfortunately, the paint soon began to chip off. I can easily remove it with my fingernail (or any other rigid object). The reason for this, so I believe, is that the case was not only painted, but also waxed at the factory.
Thus, unless the case is bare plastic (like that on my old Zenith Data Systems 8088, or the Dell I'm typing this on now), one should sand the existing surface. It probably isn't necessary to sand to bare metal, just enough to cut away the wax. Alternately, you could sand it bare, use a primer made for use on metal, and paint from there.