Do a clean install. Insert your DVD, when asked if you want a clean or custom, choose advanced options. In advanced you have the option to format, do it. Then choose custom install and you should be ready to go. Before attempting anything of this magnitude, always make adequate back up.
Dont give your key and uncheck the box asking to activate automatically. Give your number after you have tried the OS, loaded your programs and are sure that you like it. You have 30 days to give your number. After you are activated make a copy of the Drive and put it in a safe place. Even in the event of a major problem, you will not have to reinstall your programs or reactivate.