I am not sure what you are getting to, Are you installing Windows 8 on a seperate machine, and then wanting to swap the HDD out on a windows 7 machine, because this wont simply work (You could make it work but there is a long winded process involved that I am not going to go into here as you are by no means ready for the process)
Just remove Windows 7 HDD, Install Blank HDD, Then Install Windows 8!