A little more on my problem.
The hard disk itself is fine. The motherboard is toast. The logical solution is to replace the motherboard and go to work.
Unfortunately that runs square into Microsoft's copy protection fetish. If you replace the motherboard the operating system thinks its been loaded on a new computer, so it won't work until it is reauthorized by Microsoft. To get it reauthorized you need the OS serial number -- which I didn't get with the computer (remanufactured). So I'm stuck.
This has been very frustrating.