Well, not a do-over, exactly, but the damage has been contained.
The drive is probably toast, but Bob's brother had made him a bootable Linux CD that saved the day. You can boot from the disc on any computer and try out the operating system without installing anything at all.
My HD-less computer was looking for things to boot from, so I fed it the disc, which booted up fine.
I didn't know when I tried it whether A) The hard drive would be accessible at all, and B) Linux would be able to see files on a Windows-formatted drive, but the answer to both questions turned out to be a very happy Yes.
And Bob has a 16-GB memory stick, and it only took two trips up and down with that, and now all my files are safe on a combination of these two computers - turns out this new one only has a wee little hard drive, so all my stuff wouldn't even fit on it.
So now I need to acquire a new hard drive - we've already discovered that I can get about twice the Gigs for about half the price if I bring one in from the States (assuming the bringing itself doesn't get too costly), but I can also get one here if I get too impatient.
So: Long day, happy ending.