static void Main()
        {
            bool runone;
            System.Threading.Mutex run = new System.Threading.Mutex(true, "xinbiao_a_test", out runone);
            if (runone)
            {
              run.ReleaseMutex();
              Application.EnableVisualStyles();
              Application.SetCompatibleTextRenderingDefault(false);
              Application.Run(new Form1());
            }
            else
            {
                MessageBox.Show("已经运行了一个实例了。");
                //如何获取这个已经运行的实例呢？？？？
            }
        }

System.Diagnostics.Process[] allProcess = System.Diagnostics.Process.GetProcesses();
            foreach (System.Diagnostics.Process thisprocess in allProcess)
            {
                string processName = thisprocess.ProcessName;
                if (processName.ToLower() == "进程名字")
                {
                    try
                    {
                        //LZ在这操作你要操作的东西
                    }
                    catch (Exception e)
                    {
                        MessageBox.Show(e.Message);
                        return;
                    }
                }
            }