Add to shortcut to Windows start-up folder:
Environment.GetFolderPath(Environment.SpecialFolder.Startup)
Or add to registry, something like this:
RegistryKey add = Registry.CurrentUser.OpenSubKey("SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Run", true);
add.SetValue("Your App Name", "\"" + Application.ExecutablePath.ToString() + "\"");
You can change CurrentUser to LocalMachine if you want it to run with every user. Thanks to Aidiakapi.