I now strongly believe that my problems were caused because at the same time I did Ccleaner reg cleaning to speed up my computer, I also uninstalled Dropbox.
I have now
1) re-installed Dropbox and
2) signed back into it, and..........
the problems I have been experiencing have been reversed. To recap - the problem was that certain programs could not "see" files in my downloads folder, such as Chrome, when I wanted to attach a file to an email in my Yahoo email account by clicking on the papercllip - my downloads folder would appear but no files were visible, simply the words "Working On It" were visible.
I"m not 100% sure that dropbox is to blame for what happened. However, searching Google for phrases like:
I uninstalled dropbox and my computer is not working right "windows 10"
and
I uninstalled dropbox "downloads folder" "windows 10"
did NOT lead me to any posts resembling the thing that happened to my computer.
I definitely do want to speed up the boot time on my PC (7 minutes seems like a long time!) , but I actually may just ignore the problem for the next few months. Having spent 7 days worrying that I damaged my system, my current view is that it's just not worth it.