Problem solved. The answer is below:
"Here are the instructions.... usual registry editing "warning" applies.
Always backup the registry before making changes.
Launch Regedit.exe and navigate to the following branch:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\VolumeCaches\Thumbnail
Cache
Create a new DWORD (32-bit) value named StateFlags
Leave the value data to 0
Exit the Registry Editor."
Ramesh Srinivasan, Microsoft MVP [Windows Desktop Experience]
C.B. said:
Thank you very much for your assistance. I tried what you suggested but it
didn't work. The checkmark remains in the "Thumbnails" box. I haven't
rebooted yet. It's quite possible the result will be different after the
reboot. I'll let you know.
C.B.