Are you installing a new OS or downgrading to the previous OS?
If you're installing a new OS it's a simple as Vengeance said, format c: /s
Then pop the disk for the new OS and install it. Back up your files. The disk for the new OS should even give you the option to format your c drive without using dos to format c:. Just pop the new disk in and see how it goes.
If you're downgrading, XP gives you the option to do this but only if you upgraded a previous Windows OS and told XP to save this restore point.