If only mom's computer were all dead it would have been easy. Do the stupid HP system restore which "Nukes it from space, it's the only way to be sure."
But no, it's only almost dead -- the Windows folder was gone, but the Users and other folders were still there. I was a little disturbed to see the "Startup Repair Failed" -- and so was the guy at HP support, he said "Oh, that's bad."
Unfortunately, as I mentioned above, HP only provides a restore-to-factory-conditions method, no way to reinstall Vista, so far as I could tell. So I called up HP, went through the phone tree, and the guy was actually very good. No, he couldn't help me, but he (a) acknowledged that the options were bad, (b) didn't try to walk me through his script once I demonstrated that I was indeed the master, and generally tried to be helpful.
Me: "Can I write to a USB drive from the Command Line mode?"
HP: "Yeah!" (too bad my parents have no thumb drives around, more on that below)
Me: "What about writing a CD or DVD?"
HP: "That should work too."
Me: "Uh, what command line commands work to write to CDs?"
HP: "Oh. I guess you need Windows for that."
I thought I'd have to drive home to get a USB stick, but then I remembered my Crackberry has a 4GB microSD in it... and luckily we found a cable. It looks like Windows' recovery system only has USB 1.1 drivers, 'cause it was slower than molasses would be outside right now.
So dozens of reboots and "Please wait for updates to install" messages, downloading the antivirus from AT&T, getting HP drivers for her printer, restoring the cruft from the Blackberry... four hours at Mom's house, and probably a couple more to come.
It could have been worse... but as I said, "all dead" would have had me out of there in an hour, but them a lot more miserable.