Program.cs

using System;
using System.Diagnostics;
using System.Reflection;
using System.Threading;
using System.Windows.Forms;

namespace TimeWidget
{
    static class Program
    {
        /// <summary>
        /// Главная точка входа для приложения.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.ThreadException += new ThreadExceptionEventHandler(Exception);

            /*Process pr = RI();
            if (pr != null)
            {
                MessageBox.Show("Приложение уже запущено!", "Приложение запущено!");
                return;
            }
            else
            {*/
                Application.Run(new Form1());
            //}
        }
        private static Process RI()
        {
            Process current = Process.GetCurrentProcess();
            Process[] pr = Process.GetProcessesByName(current.ProcessName);
            foreach (Process i in pr)
            {
                if (i.Id != current.Id)
                {
                    if (Assembly.GetExecutingAssembly().Location.Replace("/", "\\") == current.MainModule.FileName)
                    {
                        return i;
                    }
                }
            }
            return null;
        }

        static void Exception(object sender, ThreadExceptionEventArgs e)
        {
            MessageBox.Show(e.Exception.ToString());
        }
    }
}