Hy, Today i had the same problem. My antivirus detected some virus, and deleted some file ... After restarting, i coudn't log in ( administrator or my account), win 8 show only SING OUT... I tryed to press F8, Schift +8 nothink. I read a lot of about " HOW TO ENTER SAFE MODE" , but i can not enter in WINDOWS!!!! And then i insered my Windows 8 Instalation USB stick, and there is ---> Repair your computer. And then i made a repair windows 8, without lossing anny settings.