hmmm
Boot from your install disks. Make sure you have everything you want off of it. From here, open the disk utility. You can find this in the top bar. It will take a few seconds to open up, this is ok. From the bar on the left select the drive you wish to erase. Then in the main area, There is a bar with some options, choose the second one. Then from here choose the secure erase option. If you are crazy about secure information, choose 35 pass, otherwise just 0 out all data will be sufficient. Once it is finished, quit out of the utility, and start the install.
Good luck and I hope you get a good deal on your mac book!
as for the registration screen, unsure, i would open it up and close at registration screen reopen and see if it shows up!
if so i bet it would show up again..
good luck