The original Galactic Civilizations for OS/2 was somewhat notable in that it was the first commercial 32-bit game.
While there are some games that offer 64-bit editions, most of them also include a 32-bit version. In the case of GalCiv III, it is natively 64-bit. That means no 32-bit version. You must have a 64-bit version of Windows Vista, Windows 7 or Windows 8 (most people these days do but not everyone).
So before you join up, make sure you actually are running a 64-bit version of Windows.