From void Main_Screen_Paint(object sender, PaintEventArgs e) it is clear that your application is Windows.Forms. The RadialGradientBrush however is a WPF component. That's just not compatible. 
Personally I think it borders on silly that the WPF namespaces start with System.Windows, but such is life.
                  Henk Holterman
                   2010-08-18 21:28:09