[STAThread]
    static void Main()                  // args are OK here, of course
    {
        bool ok;
        m = new System.Threading.Mutex(true, "YourNameHere", out ok);

        if (! ok)
        {
            MessageBox.Show("Another instance is already running.");
            return;
        }

        Application.Run(new Form1());   // or whatever was there

        GC.KeepAlive(m);                // important!
    }

or

static void Main()
{
  bool mutexCreated = false;
  System.Threading.Mutex mutex = new System.Threading.Mutex( true, @"Local\slimCODE.slimKEYS.exe", out mutexCreated );

  if( !mutexCreated )
  {
    if( MessageBox.Show(
      "slimKEYS is already running. Hotkeys cannot be shared between different instances. Are you sure you wish to run this second instance?",
      "slimKEYS already running",
      MessageBoxButtons.YesNo,
      MessageBoxIcon.Question ) != DialogResult.Yes )
    {
      mutex.Close();
      return;
    }
  }

  // The usual stuff with Application.Run()

  mutex.Close();
}