A global mutex across the machine

final-startup.cs

public class Program
    {
        static readonly Mutex mutex = new Mutex(true,
            "YourCompany-YourApp-SingleInstance-" + Environment.UserName);
        [STAThread]
        public static void Main(string[] args)
        {
            if (mutex.WaitOne(TimeSpan.Zero, true))
            {
                try
                {
                    // now we start the real application.
                    // in this case a WPF-app.
                    new App().Run();
                } finally
                {
                    mutex.ReleaseMutex();
                }
            } 
            else
            {
                FocusSwitcher.SwitchToCurrent();
            }          
        }
    }

FocusSwitcher.cs

public class FocusSwitcher
{
    [DllImport("user32.dll")]
    private static extern bool ShowWindowAsync(HandleRef hWnd, int nCmdShow);
    [DllImport("user32.dll")] 
    private static extern bool SetForegroundWindow(IntPtr hWnd);
    private const int SW_MAXIMIZE = 3;

    public static void SwitchToCurrent()
    {
        var hWnd = IntPtr.Zero;
        var thisProcess = Process.GetCurrentProcess();
        var processes = Process.GetProcessesByName(thisProcess.ProcessName)
                                .Where(p=>p.Id != thisProcess.Id);
        foreach (Process runningProcesses in processes)
        {
            // Get the first instance that is not this instance, has the
            // same process name and was started from the same file name
            // and location. Also check that the process has a valid
            // window handle in this session to filter out other user's
            // processes.
            if (runningProcesses.MainModule.FileName == thisProcess.MainModule.FileName &&
                runningProcesses.MainWindowHandle != IntPtr.Zero)
            {
                hWnd = runningProcesses.MainWindowHandle;
                ShowWindowAsync(new HandleRef(null, hWnd), SW_MAXIMIZE);
                SetForegroundWindow(hWnd);
                break;
            }
        }
    }
}

global-mutex-with-username.cs

var mutex = new Mutex(true,
            "YourCompany-YourApp-SingleInstance-" + Environment.UserName);

global-mutex.cs

var mutex = new Mutex(true,
            "YourCompany-YourApp-SingleInstance");

start-up-app.cs

public class Program
    {
        static readonly Mutex mutex = new Mutex(true,
            "YourCompany-YourApp-SingleInstance-" + Environment.UserName);
        [STAThread]
        public static void Main(string[] args)
        {
            if (mutex.WaitOne(TimeSpan.Zero, true))
            {
                try
                {
                    // now we start the real application.
                    // in this case a WPF-app.
                    new App().Run();
                } finally
                {
                    mutex.ReleaseMutex();
                }
            }            
        }
    }