Before you begin, make sure you backup all the inforamtion you need such as personal document, web browser bookmark/favorites, emails, pictures, videos and any other importnat files worth keeping.
Just like tttt03 said, change boot sequence to boot from CD with your Windows setup CD. It gives you a chance to repartition and format (quick format to save you time) your hard disk then install the OS. Most of the time, using all default settings is fine.