You probably want to put that code (or rather the version below) into the form's Paint event.
System.Drawing.SolidBrush myBrush = new System.Drawing.SolidBrush(System.Drawing.Color.Red);
System.Drawing.Graphics formGraphics = e.Graphics;
formGraphics.FillEllipse(myBrush, new Rectangle(0,0,200,300));
myBrush.Dispose();