Cannot Write Files After Resuming From Sleep Without Lock

I have two Windows 8 UltraBooks: a Sony Vaio SVT13126CXS, and a Samsung NP540U3C-A01UB. Both were setup fairly simply in December 2012, except that they both had the Intel/McAfee anti-theft software installed then. My kids use them for school work and typically put them to sleep when not in use. To our great frustration, they have found since about June 2013 that they are unable to write files anywhere on the hard drive when they get to school (they can write to a FAT32 USB stick). I suspect this is due to some Windows Update, but I don't know which one.

To fix this, I first tried explicitly giving the standard users ownership and full control of the My Documents tree, but that didn't work. Then I tried turning the UAC prompts down or off, but that didn't work (no UAC prompt/screen dimming/beep ever appears, anyway). Next I tried deleting their user accounts and files and re-creating their accounts as members of the Administrators Group, but even that didn't work. For instance, when they try to save a new MS Word document, a message box pops up saying something like "you need to ask an Administrator for permission" - which is bizarre, since they are Administrators! Trying to save on the Desktop or C: does not work either. I tried switching from sleep to hibernation, but that didn't work.

It finally occurred to me that the users were apparently losing part of their account privileges when the PCs were resuming from sleep. So I activated the password lock screen and sure enough that fixed the problem! Now when the PCs awake from sleep the users can enter their password to unlock the screen and then write and delete files at will. This is one of the most bizarre problems I have ever seen with Windows. I am posting it here for two reasons: underlying Windows 8 problem in this non-Pro version? So workaround (enable the password lock screen upon resume)