Blue screen of frustration...

Alright so i found this site with my first problem which was my mac came to a grey screen with a flashing folder and it was indeed the HD that failed. I took it to apple care and got a new one, I had the computer backed up on an external drive and everything looked like it was all fine and dandy, except it was back to snow leapord ( I was on lion). When I got to the log in screen absolutely none of the accounts passwords would work, so i decided to shut it down and try again. It came up to the apple grey load screen and then stops on a blue screen. Its not my computer but my dads but I can't think of anything he would have done different then me but he's not coming home till tomorrow so if i can get this resolved tonight it would be greatly appreciated. Thanks