Example using Application.Run() is below; if you are just going to .Show()
the form, then obviously lose the "using" or it will commit suicide too
early.
Note also the use of Load for the once-only startup event.

Also note that the below is NOT enough to hide it from [Alt]+[Tab] (try it
and see). You can achieve this using [DllImport] and a few Win32 calls
(possibly also in native C#)... but my example seems to have disappeared.
Let me know if you need this...

(perfect for hosting MP for a background route calculator... ;-p)

Marc

           using (Form f = new Form())
           {
               f.StartPosition = FormStartPosition.Manual;
               f.Location = new Point(-500, -500);
               f.Size = new Size(200, 200);
               f.ShowInTaskbar = false;
               f.Load += delegate { f.Visible = false; };
               Application.Run(f);
           }

While this may work, I think the best solution is not to show the form
at all.  If the OP doesn't want the form visible, then he shoule not
show it!!

You can call Application.Run without showing a form.  I presume that
the OP is trying to have an app that starts without showing a form and
perhaps has an icon in the system tray.