Run Only One Copy Of Application

How about using a Mutex? This should also work for Terminal Services sessions. Use in Program.cs

static Mutex mut;

try
{
    bool isOwned = false;
    mut = new Mutex(true, Application.ProductName + " MUTEX: {53A4988C-F91F-4054-9076-220AC5EC03F3}", out isOwned);

    if (!isOwned)
    {
        MessageBox.Show(Application.ProductName + " is already running on this machine.",
                         Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
    }

    Application.Run(new Form1());
}
finally
{
    try
    {
        mut.ReleaseMutex();
    }
    catch (ApplicationException)
    {
    }
}