It's not sure what you mean with 'the program', but if you want to limit your application to one instance then you can use a Mutex to make sure that your application isn't already running.
[STAThread]
static void Main()
{
Mutex mutex = new System.Threading.Mutex(false, "MyUniqueMutexName");
try
{
if (mutex.WaitOne(0, false))
{
// Run the application
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new MainForm());
}
else
{
MessageBox.Show("An instance of the application is already running.");
}
}
finally
{
if (mutex != null)
{
mutex.Close();
mutex = null;
}
}
}