An informative article I found at ars technical:
How Best Buy’s computer-wiping error turned me into an amateur blackhat
How to Reset Windows 8 and Windows 8.1I always get a little thrill opening a new gadget. The computer looked like it had never been touched and all the paperwork was still in sealed bags. There was even a slip of paper in the box with the ID of the tech who cleaned and certified the unit.
So it surprised me when I booted up and saw someone else’s name and Hotmail address at the login prompt. So much for like-new!
As I stared at the full name and e-mail address of the previous owner—let’s call him David—I wondered. Could I get into this computer another way? It was mine after all. And how much more could I learn about him? How bad of a mistake had the store made?