Thanks for those!
My USB drive is FAT - it has to be, because I also write to it from time to time with Knoppix for disk data recovery, and Knoppix isn't allowed to use NTFS. MS made them remove the capability. I will run the drive against Avast before we go further.
Yes, I changed the filename from proquota.exe to proquota.was under a CMD prompt while it was "shut down" - and it reappeared when I rebooted.
Gillian's son e-mailed me this evening to tell me her PC is seriously ill again, and that she's ordered Win 7.
The children are 28, 26, 24, and 19 and now all have their own laptops. This PC will have two users - Admin and Gillian.