Oh, in my case it is my own machine, so the decision making was slightly easier :)

I didn't use Win10 tool - it was probably too late for that.
I was able to boot into safe mode, and back up the data I needed to the second HD (the system is installed on a SSD drive).
Then I used...

Unlikely. I've scanned the drive while still on Win10, and there were no errors. Now, that I reverted the system back to Win7, it is running just fine on the same drive.

This one kb3176936

I gave up on this - restoring the system to Win7.

In Safe Mode, under the MS updates I saw a few from 9/26/16 (not 9/29).
Removed most of them, but couldn't uninstall 1 - the "uninstall" button...

+1
I am another "coincidence" with Dell Latitude E6520, whose laptop won't boot with this error since yesterday (9/29/16)