I spent many hours tonight trying to get my parents' PC back into shape thanks to Remote Assistance. Before giving up I did manage to figure out what had happened: somehow, when my dad ran VirusScan, it caused my mom's account to be swapped with the Guest account. I don't have the slightest idea how that happened. Logging in under her name actually logs in the guest user, but it still has her name. What used to be her user registry hive is now under HKEY_USERS\.DEFAULT. It's mind-boggling. I have limited knowledge (but some) about the NT user and login system, and this defies all knowledge I have of it. I don't know how a user account and the guest account can just become swapped like that.
But, the good news is that the files are all still there; they're just not where they used to be... which is good, because my parents don't back up documents at all, and my mom only backs up her photos every month. If it weren't for Digital Image Library imploring her to do so each month she'd never do it, like probably the majority of the world.