I would say just copy your User Folder to your external HD and any apps you have added. Then insert your restore disc. Restart the computer, hold C, then after selecting language, click on the Utilities Menu, then select your HD and go to Erase, then click on Options, and select "write 0s to Disk". Click Erase and enjoy. Once done, simply Quit Disk Utility then reinstall the OS (just so it will boot when they get it).