Almost every time I update my Windows 8.1, the whole thing breaks. After login (which takes an unusually long time), the taskbar will appear with only the Start button (mapped to Classic Shell). The desktop background also appears. About half a second later, the screen flashes black, and the process repeats forever. Ctrl-Alt-Delete does bring up the security options, etc. If I log out and log back in, it repeats. If I log in with a different account, same thing. Sometimes I can reboot and it is fixed, but not anymore. Everything works except the Desktop, however I can open the Task Manager and it will stay open. I cannot run a command from it though. I could fix it with a System Restore, but the latest one was saved when updates were ready to install. So I restore, it reboots, updates, and I'm back where I started. I cannot access the Metro interface because of mapping to Classic Shell. The charms do not work. I think it doesn't have enough time to show them before it reloads the Desktop. I need this fixed, because my college campus blocks computers without current updates from connecting until they're up to date. Please help!
try removing classic shell, either by booting to safe mode (which windows 8 has made much harder to access) or by booting a live usb and removing the classic shell registry entries w/ chntpw 's registry editor.
Additional information added December 6, 2013: You might be able to get away with reinstalling classic shell after updating, but I would make a disk image backup before attempting that. Just to be safe!
This happened to me a while back. I didn't update to 8.1 though, it was just a minor update inside windows 8.0 (I know it's technically NT 6.2.9200). I hope it never happens again to either of us!