SOLVED - at least for now.
I had the same problem with no checkbox to clear and no warning. It looks like different people at different times are getting different screens.
Anyway I did a system restore to get rid of the Mcafee, and then after a lengthy-ish search I found this page with a link to these standalone installers.
I selected the MSI Installer for Windows Plug-in based browsers, and that did the trick. I now have Flash support in Firefox, Chrome, and Opera with no Mcafee on my computer. Interestingly, I have to click a play button in Opera (at least for the site I checked), but that's no big deal. Flash in IE is not working so I suppose I'd need to download a different installer, but as I rarely use IE I don't really care.
Here are those links in the clear:
www.pretentiousname.com/flash_links
www.adobe.com/products/flashplayer/distribution3.html
Hope this helps someone.
The credit goes to Leo at pretentiousname.com