Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    bool result;
    var mutex = new System.Threading.Mutex(true, "MakeThisAHighlyUniqueString", out result);

if (!result)
{
    MessageBox.Show("Another instance is already running.");
    return;
}           
Application.Run(new MainForm());
GC.KeepAlive(mutex);