So i am going to be helping a friend build a PC, He currently has a pre-built piece of crap that has pretty much stopped working, He has 2 Hard drives in his current pc and the pc has windows 7 on it already but it can't be transferred over to the new build. One hard drive has his OS and i'm not sure what else on it and his other hard drive which is new has files on it. My question is what do i have to do to install a new copy of Windows 7 on the hard drive that currently has it on there and could the second one just be plugged in after the computer has posted and done driver updates without any problems with files still intact? I would appreciate any help i just got done building my PC but i had new hard drives so this is a new one for me.