After two evenings of boot disks, operating system CDs, external drives, SATA drives balanced precariously atop an open case, and a couple of hard ciders, it appears the great computer meltdown of 2010 has been repaired.
Sadly, after all the effort, I still don’t know what was wrong. The computer stopped booting without an error, so I tried to fix it with a variety of different tools. Some of them may have introduced other errors, or exacerbated the original error, and somehow it all ended up booting again around 6pm today.
Things which I tried which did not help: fixmbr, fixboot, copy partitions to a spare SATA drive I have lying around (waiting for that new build I’ve been planning for nearly a year now), copying NTLDR, hiding and unhiding partitions, making partitions active and boot, and pulling out hair.
Things which I think led to the fix: editing the boot.ini file via a Linux boot disk to point to partition(2) instead of partition(1), ensuring the recovery partition does not get assigned a drive letter in XP. And possibly the cider.
And this is why I have several USB drives about, as well as why I experiment with live distros on USB keys so I’m not completely flummoxed when everything goes pear shaped. I still don’t trust this machine, though. Flaky like croissant dough.