I am far from an expert on this topic, but my understanding (admittedly, just from Wikipedia) is that there is a 64-bit edition as well.
well, originally, windows 2003 has only 32bit edition, and lately Microsoft add a windows 2003 64bit extended edition.
Why?
because of GD, IIRC, the older GD (like 2.5x) only run correctly on windows 5.20, 5.22. he can install the same version GD on windows 2003 to new windows to avoid API changing.
I am trying to improve my English skills, if you see a mistake please feel free to reply or /msg me a correction