I apologize, Alan_B, for not having done a better job of posing my question. That free-space figure I used is hypothetical ("Let's say....") as I was looking for an educated guess on how long it might take CCleaner to wipe free space. Let's forget the "500 Megabytes" and use whatever figure everybody's more comfortable with: if I've [insert acceptable figure here] of free space, how long should it take CCleaner to complete a one-pass wipe? It seems a reasonable question to me; and any estimates will be greatly appreciated.
Marmite, are you an acquired taste? (Sorry, I couldn't help myself.) Anyway, thanks for the sdelete.exe link. Well, I clicked on "Open" and up came a command-line box along with a EULA box; and when I accepted the EULA both boxes vanished. Upon clicking "Open" again, the dreaded black box appeared for a split second and vanished once more. Maybe it's busy doing something behind the scenes? I'm not a big fan of command-line stuff 'cause it's user unfriendly - for users like me, anyway. I'll wait and see if it reappears, or something.
=^..^=