Program.cs

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Threading;

namespace FileChangeNotifier
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        static Mutex mutex = new Mutex(true, "FileChangeNotifier");
        [STAThread]
        static void Main()
        {
            if (mutex.WaitOne(TimeSpan.Zero, true))
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new frmNotifier());
                mutex.ReleaseMutex();
            }
            else
            {
                NativeMethods.PostMessage((IntPtr)NativeMethods.HWND_BROADCAST, NativeMethods.WM_SHOWME, IntPtr.Zero, IntPtr.Zero);
            }
        }
    }
}