>>959 こんなふうにして下さい
unit Unit1;
interface
uses　Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,　StdCtrls;

type TForm1 = class(TForm)
　　Button1: TButton;
　　procedure Button1Click(Sender: TObject);
　　procedure FormCreate(Sender: TObject);
　private
　　procedure NewButtonProc(var Msg: TMessage);
　public
　　OldButtonProc:TWndMethod;
　end;

var Form1: TForm1;

implementation

{$R *.DFM}
procedure TForm1.NewButtonProc( var Msg: TMessage);
begin
　with Msg do
　 case Msg of
　　WM_RBUTTONDOWN: Msg := WM_LBUTTONDOWN;
　　WM_RBUTTONUP:　 Msg := WM_LBUTTONUP;
　 end;
　　OldButtonProc(Msg);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin　　 ShowMessage( 'こんにちは' );
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
　OldButtonProc:= Button1.WindowProc;
　 Button1.WindowProc:= NewButtonProc;
end;

end.