[Satcomer]

You might want to list the Operating System that the computer is running. I say this because in the old OS 9 days just copying system files to a new disk would be enough. However O S X is based on BSD Unix so just copying files will not due. In your situation the best bet would have been a external Firewire hard drive enclosure and connected that to the Mac with the files on it. Then put the blank hard disk in the enclosure. Next would be to use Disk Utility in /Applications/Utilties/ and formated the disk Mac OS Extended(Journaled) . Now to move the files you cant just copy the system over hoping for the best because it needs to be "blessed". a cool free product to "clone" the drive (and make it boot able) is called Carbon Copy Cloner to move the system and files to the new drive. Then look at this howto tutorial on the insulation to put the new drive into the iMac.