Two HDs, each with OS

[CrazyEyez]

I'm trying to fix a friends PC. Windows is all [censored] up. I want to take a new drive and install windows, then add in his old drive and copy over all the files he needs. Is there anything I need to know? Will things get jacked up when I try to boot the machine with two drives that each have windows on them? Do I just go into the bios and tell it which one to boot from, and it will ignore the other (as far as the OS is concerned)?

Thanks.