Проверьте на инф - цель c

static void Main() 
{
  using(Mutex mutex = new Mutex(false, @"Global\" + appGuid))
  {
    if(!mutex.WaitOne(0, false))
    {
       MessageBox.Show("Instance already running");
       return;
    }

    GC.Collect();                
    Application.Run(new Form1());
  }
}
 Источник:  http://odetocode.com/Blogs/scott/archive/2004/08/20/401.aspx