Code:

procedure TForm1.Button1Click(Sender: TObject);

            var

              Rgn : hRgn;

            begin

             {Cпрятать кнопку "Пуск"}

              Rgn := CreateRectRgn(0, 0, 0, 0);

              SetWindowRgn(FindWindowEx(FindWindow('Shell_TrayWnd', nil),

                                                   0,

                                                  'Button',

                                                   nil),

                                                   Rgn,

                                                   true);

            end;

            procedure TForm1.Button2Click(Sender: TObject);

            begin

             {Показать кнопку "Пуск"}

              SetWindowRgn(FindWindowEx(FindWindow('Shell_TrayWnd', nil),

                                                   0,

                                                  'Button',

                                                   nil),

                                                   0,

                                                   true);

            end;

            procedure TForm1.Button3Click(Sender: TObject);

            begin

             {Запретить кнопку "Пуск"}

              EnableWindow(FindWindowEx(FindWindow('Shell_TrayWnd', nil),

                                                   0,

                                                   'Button',

                                                   nil),

                                                   false);

            end;

            procedure TForm1.Button4Click(Sender: TObject);

            begin

             {Разрешить кнопку "Пуск"}

              EnableWindow(FindWindowEx(FindWindow('Shell_TrayWnd', nil),

                                                   0,

                                                   'Button',

                                                   nil),

                                                   true);

            end