I have installed Win 10 as an upgrade on a seperate partition to my Win 8.1. Win 10 is x64 version and Win 8.1 is 32 bit. As far as I can tell this puzzle started when I upgraded the x64 partition to Win 10. Now when my lock screen appears and I click the mouse for the password box there is a back arrow next to my user name. Pressing this gives me a black screen, then my user name reappears without the back arrow and I login. I am using the same MS account for Win 8.1 and Win 10 TP. This doesnt happen on Win 10 lock screen/password screen. Is there anyway I can get rid of this arrow, just a niggle I know but would be nice.