After several tests and false starts, I finally opted for a Windows OS reset, keeping my files.  This appeared to work.   When I first saw the login screen after the reset, it had the same message about my PIN being unavailable. This time, however, the option to reset my PIN worked and I was able to create a fresh one. I've now logged in and am beginning the painful task of re-installing and configuring my apps. Whether the problem lies with Dell or Microsoft, I still think it's unacceptable that such an apparently mundane change should lead to this problem and solution. Thank you to all who read and responded.
Alienware Aurora R10, Win 10 Home, 8 Gb RAM, Warranty expires in a few days. The Dell support assist app suggested system board drivers and BIOS should be updated earlier today. Thankfully, I did them one at a time. Board drivers installed apparently cleanly and I restarted when prompted without a problem. Re-ran Support Assist and it reported that the BIOS was the only outstanding upgrade. I was on 2.1.2 and the suggested upgrade was to 2.2.4, which was dated sometime in mid-December. I fired off the upgrade, watched the system restart, then left the room for a while. When I got back, the normal start screen was there with date and time and the same the Microsoft "rocky cave at the beach" picture. I clicked with the mouse to bring up the PIN prompt and saw this message: "Your PIN is no longer available due to a change in the security settings on this device. Click to set up your PIN again.". If I click the 'Set up my PIN' prompt, the system thinks for a while, then quickly shows a blank dialog box with 'Microsoft Account' in the title bar, before taking me back to the same message. There's no option for another login method and a reboot gets me back to exactly the same position. I've Googled the issue and tried to restart the machine in Safe Mode, but I just end up with the same login screen and message, with no option to login using password. In desperation, I've rolled back the last features update and the last quality update, even though they're related to the OS and it appears that the BIOS upgrade - a hardware feature - was the culprit. As I expected, they haven't changed the situation. I came to this site and saw some references to problems arising from earlier BIOS upgrades and wished I'd come here first, (why is hindsight always such a wonderful pointed thing?). So now I'm hoping for suggestions for a fix or a method of downgrading the BIOS back to 2.1.2. I'll keep looking for help with the latter, but if anyone has had similatr problems and knows of a fix or workaround that would enable me to login to my machie again, , I'd be grateful to hear. I'll try the Dell helpdesk, too, as my warranty still has a few days to run. Can't hurt, I guess... Steve
Aurora R10, login problem, BIOS 2.2.4