Well, after Razer's very helpful 'Ask Microsoft' response to my inquiry regarding a firmware upgrade, I checked a few forums and found that the Habu firmware cannot be updated on a 64bit OS. 32bit XP Pro allowed me to get the firmware upgraded by generally following the instructions on Razer's website. The part about needing a second mouse to complete the new hardware wizard proved correct in my case, which I find unconscionably inane.
So with the firmware updated, I was now able to install the drivers on XP Pro 64bit, and the mouse usually worked. I say usually because occasionally when I reboot or start up from hibernation the mouse is not detected - the first time this happened I tried pressing the small round button underneath the mouse (because I was under the impression that this button reset the mouse, like with the Copperhead) - this actually worked and the mouse was detected, which surprised me because on the Copperhead, this never worked.
However, subsequent start-ups have shown that this doesn't always work, and I still have to reach back there and unplug/replug the mouse on about 25-30% of my start-ups.
If I hadn't already experienced worse problems with the Copperhead, I'd be very disappointed. I've read that the DeathAdder doesn't have these problems, but at this point I'm happy with my 2500dpi A4tech X-750BF.