Boot from installation disk and select install.
type your serial, and on the next screen you'll be asked to upgrade or new install.
select new install and then select your main partition (should be the biggest one).
don't delete or format anything, if you do, your files will be deleted.
proceed to windows installation.
all your files will be saved at C:\windows_old you can restore it by copying C:\windows_old/users/yourname to c:\users/your_name.
reinstall all programs that you need.