Well, the sequel - for those who like dramas
Knoppix read Gillian's hard drive, and copied off her "stuff" to the external FAT drive. Or so we thought - the messages on screen were right. I plugged the drive into my laptop and told Avast to check it, and off it went.
I then wiped her hard drive and set Win 7 to installing. When that was done, I installed Avast, Firefox, Thunderbird, and MS Office. Then plugged in the external drive ... and her stuff wasn't on it. Whether it failed to copy (likely), or was so infected that Avast zapped the lot (less likely) we shall never know.
And then, of course, her Win 7 Home Premium Upgrade edition wouldn't "validate" online because it wasn't installed on top of a previously licenced copy. I phoned the MS activation line and got a charming Indian call centre person who told me I would have to wipe the lot, reinstall XP Pro, then install Win 7 over that. We didn't, and aren't going to. Next time she and I are in the same county we will phone MS and hopefully get the UK helpdesk who will give us the validation code to enter.
Or I may just use the registry tweak I've been sent.
There followed an "interesting" hour or two recovering stuff from various places. We recreated most of her e-mail address book from the past two weeks' incoming mail on her server, plus my address book (we share many friends).
Next session will be about an external backup hard drive (or maybe a memory stick).
She now knows never to open an e-mail with an attachment, and not to turn off Avast.
The good news is that the machine is dramatically faster than it was before the procedure.