System.Drawing.SystemIcons, although note that not all icons are included and at least the Shield icon is incorrect on Win 7 / Win 2008.

            wndclass.hIcon = Win32.LoadIcon(IntPtr.Zero, new IntPtr((int)SystemIcons.IDI_APPLICATION));

To load system icons, provide IntPtr.Zero for hInstance, and use one of the values of SystemIcons for lpIconName

public enum SystemIcons

     Public Enum SystemIcons