I have 3 people set up with their own password protected accounts on my laptop. Mine is the Admin and each of the kids are Standard (non-Admin). For some unknown reason, neither of the kids can sign on to their accounts now- and this is the second time for the boy.
I just updated and currently running a full scan with MS Defender and will report findings in a couple of hours or so.
Both accounts are now accessible after a Windows Update. Unlike Vista, which would update/reboot by itself whenever "patch Tuesday" came around, W8 does exactly that- it waits and informs you that it will reboot itself in 2 days to finalize updates if you don't save your work and do it on your own before then.
So now I don't know if there was something wrong with their accounts and was fixed by the update(s), or maybe there is a feature that locks those accounts out when there are updates pending.