My first attempt at a rebuild was, err, abortive. In fact, I got so stuck, I ended up talking to a chap at Microsoft PSS. After all, what else do you do when both Google and your wife who writes help magazine columns for a living and has more PC internals books than anyone else around can't help? There I was with a machine that kept asking for authorisation and then failing to load me into a user shell. No way of getting past the problem, and no way of safely replacing the corrupt code.
The PSS engineer found this quite unusual... To the extent of telling me "I didn't know that could happen." Unfortunately it didn't seen to fixable, as something had corrupted some really secure code.
When I break things, I really break them.
We suspected the failing hard disk, but it was a rather unusual failure mode. I suspect what was failing was not the usual mechanical systems, no catastrophic head crash or motor seizure. Instead, something was happening to specific sections of the drive platters - something that was slowly spreading. However that did mean that if I could install the OS on a new drive, there was a chance I could extract the files off the slowly dying platters.
Luckily it worked.
Still, after shuffling new and old hard disks around the machine I seem to have a PC that's working again, along with a copy of all my files (including my recently updated accounts).
Let's just hope it lasts a while...