Since I use both 32 and 64, I just chose "latest". My box is XP Pro 32, wife has been XP Pro 64 for a loong time. I just got a new box, Dell Precision 490, and was handed the original Dell discs, so it is now XP Pro 64, all fed to the same big monitor.
My Nikon Camera download software will not work with XP 64 bit....not because it's 64, but because it looks for SP3.
Since XP 64 never had a SP3, it won't even load.