To clarify...
I was using a different keyboard... But, it was not likely a keyboard issue -- as I was also using F8, Alt-F8, Alt-F9, etc. in ZTree, around the same time (and they were all working OK) -- while trying to get Alt-F7 to respond, but Alt-F7 was entirely "dead" in ZTree.
Then about an hour after that problem appeared (shortly after I posted my Comment - id=119644), Windows froze up entirely. I killed explorer.exe and attempted to restart it, but it would not respond, and I was forced to power-off the computer. Now, looking at the Event Viewer's System and Applications Logs, that entire time frame (for several hours) is "blank".
The expected "Critical" Error, Event ID 41, was NOT even recorded by Windows:
"Microsoft-Windows-Kernel-Power -- The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly."
Cause / diagnosis : Unknown...