I have a surprisingly simple solution that worked for me after everything else got me nowhere. I'd uninstalled some windows updates to try to isolate which might have caused corruption to one system file showing up in sfcdetails - without creating a restore point (stupid), not to mention the corrupt file was part of a service I have disabled anyway! Long story short - after uninstalling the updates, upon next reboot, whether normal or safe, cleanboot or not, and after numerous sfc /scannows and chkdsks from safemode command prompt and F8-system restore, explorer would crash (or restart after closing the error window, faulting ntdll.dll btw), nonstop. And task mgr was a glimmer of hope until I found that even though I'd found out how to get Windows Updates - the install would hang, eventually timing out with the error 80243004: basicly telling me that without the little update icon showing in the TASKBAR (which inconveniently needed explorer to work), I was not getting my updates/ back to normal. So....
I'd decided the only option left was an OS reinstall/ uggh. UNTIL - and here's the alt solution that in my case turned out to be the FIX:
Boot up NORMAL to the OS login screen (in my case Win7 x64) BUT DO NOT LOG IN YET - instead WAIT FOR A WHILE, 20-30+ minutes (my screen had already powered off so it was at least 15 minutes in my case). I logged in after giving it some time, and lo, all my startup apps inc A/V zipped through bootup and I was just thinking 'any second' ....but no error, so I downloaded the windows updates and restarted and have been back to normal (without waiting to login either) since!
I literally tried everything down the checklist. Must have been some program on a DELAYED START that explorer.exe needed to have going well before it started in order to be stable. Weird!
Cool if this works for somebody else, but the takeaway moral is MAKE A RESTORE POINT BEFORE UNINSTALLING UPDATES! Wish I had