A problem of Windows API

Hi friends:

It is the first time I ask for help here.

My problems is about windows API function. Below is my C# source code.When I click butten1 in windowsform1,I want the software to enter "windowers+R" and then release them, after enter "Enter". But when I run the software so many dialogue boxes come out. Please help me find out the mistakes in my code. Thanks so much.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace APItest
{
    public partial class Form1 : Form
    {

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern void keybd_event(byte bVk, byte bScan, uint dwFlags, uint dwExtraInfo);
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {

            keybd_event(91,0,0,0);//enter windkey
            keybd_event(82,0,0,0);//enter R
            keybd_event(91, 0, 2, 0);//release windows-key
            keybd_event(82, 0, 2, 0);//release R
            keybd_event(13, 0, 0, 0);//enter enter-key
            keybd_event(13, 0, 2, 0);//release enter-key
        }
    }
}