I figured out that I was smart enough to do a complete image backup of my Vista-based PC before I reverted to XP. And luckily, I imaged the PC to an external hard drive. And luckily, Vista's image backup utility creates VHD files - which are used by Microsoft Virtual Server. And more luckily, one can download Microsoft Virtual Server (for free) which includes the vhdmount utility.

The upshot of all this is that I was able to get back all my previous genealogy work!