Quote Originally Posted by Max_aka_NOBODY
But if he uses an antialiased picture of the oval in an image control for example that won't work completely. Nonetheless, it wouldn't be much of a problem.
No it wouldn't. But he doesn't so it doesn't matter. I just provided a very simple solution without going in to any APIs or advanced mathematical code.

Another solution would be to simply set the BackStyle of the UserControl to Transparant, since the mouse move event will not be fired while the mouse is over a transparent part.