ALTERNATE SOLUTION:
I have had one pesky THUMBS.DB file, for 4 years (based on its file date).
I've tried deleting it multiple ways, including using UNLOCKER, without success.
Windows had always claimed it as 'in use' even though it had no business to be 'using' it.
FYI - Renaming to .dat (or anything else, or renaming the folder) cannot work if Windows is 'using' the file...
But I just tried using Task Manager to END TASK all open applications, including several minor Windows apps, until I couldn't end any other tasks.
Unlocker was finally able to see that it was no longer in use (I still have no idea which app was 'using' it), so I asked Unlocker to delete it, and it is finally gone - no restart required, although I now need to do so to get all my background apps again...