C#

ProcessStartInfo psi= new ProcessStartInfo();
psi.UseShellExecute = false; //look at the discription if you need it!
psi.FileName = ???;//exe file name
psi.WorkingDirectory = ???;//path to the exe
psi.Arguments = @"xxx"; //set the arguments you whant (for more than one you have to separate with " ")
using(Process YourProcess= Process.Start(psi))
{
	YourProcess.???; //Here you can do stuff with the process if you have to.
}

rtb.Clear();
for (int i=0; i<parts.Length; i++)
{
   rtb.SelectedRtf = parts[i];
   rtb.Select(rtb.Text.Length, 0);
   Application.DoEvents();
}

// derived class from datagridview
public class MyGrid : DataGridView{

    // this method is called from "ProcessCmdKey" or "OnKeyPress", etc...
    protected virtual void ProcessEnterKeyPress() {
        MyGridProcessEventArgs me = new MyGridProcessEventArgs();
        this.OnProcessEnterKeyPress( me );
        if ( me.Handled == false ) {
            // ??? throw exception - enter key press is not handled ???
        }
        object result = me.Result;
        if ( result == null ) {
            // ??? throw exception - no result ???
        }

        this.ProcessResultOfEnterKeyPress( result );
    }
    protected virtual void ProcessResultOfEnterKeyPress( object result ) {
        // this method process result of enter key action
    }

    protected virtual void OnProcessEnterKeyPress( MyGridProcessEventArgs e ) {
        EventHandler handler = this.ProcessEnterKeyPress;
        if ( handler != null ) { handler( this, e ); }
    }
    public event EventHandler<MyGridProcessEventArgs> ProcessEnterKeyPress;
}

// contains arguments for mygrid's event
public class MyGridProcessEventArgs : EventArgs {
    private bool handled;
    private object result;

    public bool Handled {
        get { return this.handled; }
        set { this.handled = value; }
    }
    public bool Result { get { ... } set { ... } }
}

// form's class
public class Form1 : Form {
    ...
    public Form1() {
        this.InitializeComponents();

        this.myGrid1.ProcessEnterKeyPress += this.myGrid1_ProcessEnterKeyPress;
    }

    private void myGrid1_ProcessEnterKeyPress( MyGridProcessEventArgs e ) {
        // there you should show dialog or something else
    }
}