if (gui == null)
        {
            gui = new App();
            gui.MainWindow = new mainWindow();
            gui.InitializeComponent();
            IsUIOpen = true;
        }
        else if (!IsUIOpen)
        {
            gui.InitializeComponent();
            gui.MainWindow.Show();
            gui.MainWindow = new mainWindow();
            IsUIOpen = true;
        }

gui.Exit += new System.Windows.ExitEventHandler(settings_FormClosed);
gui.MainWindow.Closed += new EventHandler(settings_FormClosed);

private void Cancel_Click(object sender, RoutedEventArgs e)
    {
        presenter.Close();
        this.Close();
    }

Process myProcess = new Process();
myProcess.StartInfo.UseShellExecute = false;
myProcess.StartInfo.FileName = "C:\mysettingsapp\mysettingsapp.exe"; // replace with path to your settings app
myProcess.StartInfo.CreateNoWindow = false;
myProcess.Start();
// the process is started, now wait for it to finish
myProcess.WaitForExit();  // use WaitForExit(int) to establish a timeout