Mysteries of Windows XP: user directory spontaneously renamed

In case somebody else has the problem:

I was silly enough to leave checked the "compress files" radio button when asking for a disk cleanup last night. I don't know if that was what did the damage, or if it was the system updates, but this morning when Youngest Son logged in all his files were gone, ditto his desktop, etc. Nobody else seems to have been bothered.

His login directory had apparently changed from his name to "TEMP".

A quick googling to learn why found nothing, but windowsxp.mvps.org told how to identify the account (which has a non-intuitive numeric string in the Registry), and how to rename directories. Ah...

Using regedit I discovered that YS now had 2 accounts: one with his old account_number and one with account_number.bak . The latter had the proper directory, and the former had a directory defined by his name with the name of the computer tacked on at the end!

This was in "HKEY_LOCAL_MACHINE \ SOFTWARE\ Microsoft\ Windows NT \ CurrentVersion \ ProfileList", but undoubtedly you Windows experts knew that already. I fixed the ProfileImagePath and rebooted and it seems OK now.

I still have no idea how things went wrong. I've been responsible for managing computers at work for over 26 years (mostly Unix/Linux and VMS), and I've not seen anything quite like this before. I won't delete the .bak account if I can help it--I should change its Path to point at something harmless just in case the system decided to clean things up behind my back.