So, my old home laptop died a while ago, and my beloved wife surprised me with a new one. The new one had Windows Vista 64-bit installed, and that's what I went with.
Of course, copying over my data files from my old machine was a snap with Windows Home Server. I love my little black guardian.
Anyway, back to the topic of this post. Turns out that the reader software Sony has won't install on a 64-bit operating system - although the installer message doesn't quite say so in so many words. The workaround on http://www.vistax64.com/drivers/159137-workaround-sony-ebook-vista64.html did the trick for me - just unpack by running the installer with the -a switch, and then run from there.
New machine registered for my old account, and I've got access to all my content again.