C#

SetWinEventHook(EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_FOREGROUND, IntPtr.Zero, WinEventProc, 0, 0, WINEVENT_OUTOFCONTEXT);

private void WinEventProc(IntPtr hWinEventHook, uint eventType, IntPtr hwnd, int idObject, int idChild,
           uint dwEventThread, uint dwmsEventTime)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Collections;
//using System.Net;

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        static int countFiles = 0;
        public Form1()
        {
            InitializeComponent();
        }
        ArrayList list = new ArrayList();
     //--------------------------------------------------------// 
     //------There is something wrong with this function ------//
     //--------------------------------------------------------//
        void DirSearch(string sDir)
        {
            try
            {
                foreach (string d in Directory.GetDirectories(sDir))
                {
                    foreach (string f in Directory.GetFiles(d, "*.*"))
                    {
                        list.Add(f);
                        countFiles++;
                    }
                    DirSearch(d);
                }
            }
            catch (System.Exception excpt)
            {
                Console.WriteLine(excpt.Message);
            }
        }
      //------------------------------------------------------//  
        private void button1_Click(object sender, EventArgs e)
        {
            this.Cursor = Cursors.WaitCursor;
            listBox1.Items.Clear();
            listBox1.Sorted = true;
            list.Clear();
            countFiles = 0;
            DirSearch("c:\\");
            foreach (string s in list) listBox1.Items.Add(s);
            listBox1.Items.Add(" --> Number Of Files = " + countFiles);
            this.Cursor = Cursors.Default;
        }
    }
}