Setting up Windows backup, I cleared the checkbox for including new users, and I cleared all the checkboxes for two other users on my system (an administrative account, and the Guest account), leaving only my own account to be backed up. However, when I exit from the backup setup and return, I find that the other two accounts are once again selected for backup. I've tried several times to exclude them, always with the same result.

As a troubleshooting task, create another standard user, logon so that a profile is created. Setup your backup and exclude the newly created user and see if your setting persists. If so, then your original issue may have to do with the fact that the local Administrator and Guest accounts are special system accounts and not normal, user accounts created after the OS load.

Thanks for the suggestion, but the administrator account I referred to was a user-created account, not the built-in Administrator account (sorry I didn't make that clear). So I've already performed the experiment you suggest.

When you exit back up, are you exiting via the Cancel button or the red X in the dialog box's upper right corner? If so, then your settings will not be saved. You will have to actually click the Save settings and run backup button. It would be nice to have a separate Save Settings button (I just submitted feedback on that feature).

My system's button, on that same window, reads "Save settings and exit" rather than "Save settings and run backup" (and I do click that button). I'm running the official beta (6.1.7000 x64) with all updates installed.

Interesting. On both my 32bit and 64bit versions (6.1.7000), they read Save settings and run backup. Have you already run backup? If so, that would explain why you and I are seeing different buttons; I have not actually went all the way.

Yes, that explains it. I had already run the backup several times. I do recall now the "save settings and run backup" the first time I set it up.But it's still a mystery why it's not saving my settings.

Time to submit the bug report.

I already used "send feedback". Is there a more direct way to submit a bug report?(Thanks for looking into this!)

For us public beta testers, the send feedback mechanism is correct. If you are a paying TechNet or MSDN subscriber, then you would have other, more direct avenues for submitting bugs.