Thanks, Mark and Stephen, for the responses.
Perhaps I should mention that my daughter shows extreme intelligence in certain areas, although until now hacking has not been one of them. Wife and I sometimes refer to her as "13 going on 35".
Daughter is currently staying with her big sister in Virginia (she's home-schooled, no problem being away this time of year), so serious interrogation is not an option right now. Besides, she's tough enough to withstand anything I'd be willing to do to her ... and probably more.
I really doubt she changed the Guest account's properties deliberately, or could remember how she did it. I gave her Admin privileges so she could update a few apps and extensions while she was using the notebook away from home; her home desktop machine is set up for graphics applications and is always up to date, the notebook is more general-purpose and sit in the closet (no updates) for a couple weeks at a time.
Wife, notebook and I just returned from a a visit to the rest of her family in Virginia, so now I have time to wipe the Windows partition and do a fresh installation. Not my favorite thing to do, but I'll wait until the new desktop box is running so I'll still have a computer to use while I'm reinstalling and updating everything on the notebook. And maybe I'll buy a netbook or cheap notebook for Darling Daughter to use when she's visiting friends and family. And this time I won't share my Administrator password with anyone who's smart enough to ask for it ... which includes wife, daughters and maybe a couple of our goats.