I'd pop in the new HDD boot the Apple OS X DVD and start fresh. Then restore from my backups.
Sometimes folk forget a few things so you put the old HDD into some USB or Firewire case (depends on the apple model and the owner's preference) then you let them copy over the files they forgot.
If the drive is totally gone you remind them how important backups are and tell them about drivesavers.com
I have a mid(2009) MacBook pro. The question is if my MacBook pro comes on but don't load up all the way to the home screen. How can I transfer my old hard drive to my new hard drive if my computer will not completely load up to the home screen? I went to the apple store and after they ran their diagnostics they said I need a new hard drive.