Boot from WinXP CD. Start Windows installation process. When it gets to the step where you choose which partitions to install on, you have options to delete and format the partitions (and to create new ones if you like).
If you are concerned about data security, there are numerous utilities you can get to scrub a hard drive so that 'undelete'/data recovery programs will not be able to pull anything useful off of it. Please search for numerous threads on this topic.