System.Diagnostics.ProcessStartInfo start =
      new System.Diagnostics.ProcessStartInfo();
start.FileName = dir + @"Myprocesstostart.exe";
start.WindowStyle = System.Diagnostics.ProcessWindowStyle.Hidden;

[DllImport("kernel32.dll")]
    static extern bool FreeConsole();

yourprocess.StartInfo.UseShellExecute = false;
yourprocess.StartInfo.CreateNoWindow = true;