Well, I think I may have killed this bug.
I logged into XP and went directly to the Control Panel via the Start menu, avoiding any access to My Computer. I selected "Add/Remove Programs", and noticed that the NVidia driver package didn't say anything beyond its name and that there were multiple instances (with different version numbers) of all the "Windows Driver Package" items.
I uninstalled all the Windows Driver Package items, the NVidia driver, the RealTek audio driver, and the Boot Camp Services package over two (intentional, not blue-screen) reboots. I noticed that the HFS volumes mounted if the Boot Camp Services package was installed.
After the last reboot, I dropped in the Snow Leopard install CD and it auto-loaded the Boot Camp installer. This is different from what it did the first time I tried to install: that time, it popped up a window asking if I wanted to install either the Boot Camp drivers or MacBook Air optical drive sharing software.
One install and reboot later, and my NVidia card is happily running at 2560x1600 again, there's only one set of Windows Driver Package entries, and things seem okay.
My Windows guru friend always told me to uninstall the NVidia drivers and reboot before installing new NVidia drivers. Guess that should be the case for the Boot Camp drivers as well. I'm surprised Apple's installer doesn't uninstall the old drivers.