OnLButtonDown(x,y):
//if (x,y внутри эллипса){
moving = 1;
x0 = Ellipse.Left - x;
y0 = Ellipse.Top - y;
//}
OnMouseMove(x,y):
if (moving){
Ellipse.Left = x + x0; 
Ellipse.Top = y + y0;
}

OnLButtonUp():
moving = 0;