Normally you have that option when you do a "new install". If you are
upgrading then it won't format your hdd, but if you are doing a fresh
install then it will format your hdd. You have three immediate options when
you pop in the XP Pro CD. I forget 1 of them, but the other two are
"Install" and "Check system...". When you click on the "Install" option,
you should then have the option for either performing a new installation or
an upgrade. Click on upgrade and you'll be safe. Good luck!