Troubleshooting the Windows 8.1 upgrade: SecureBoot isn’t configured correctly

If you have secure boot turned off in the UEFI firmware settings, for example because you need to boot a live Linux from DVD on occasion, or want to be able to do a Windows repair from DVD without having the change the firmware settings every time, Windows 8.1 will display a watermark in the lower right corner of the desktop, claiming that

SecureBoot isn’t configured correctly

Well, Microsoft, guess what, I know that it is configured correctly. The correct configuration is to turn it off!

Luckily someone at Microsoft realized this, too, because there is a Windows patch available that will make the watermark go away. Kind of hilarious, actually.