You should be able to do this by booting from the XP Setup disks and walking through the setup procedures. However, if you only have a single hard drive with a single partition, XP may not let you do it. There are a number of utilities (freeware and shareware) that will delete all of the partitions on your hard drive regardless of the operating system and let you start over again from scratch. One of my favorites is Partition Zapper which is available for $12.00 at http://www.meixler-tech.com/pzapper/.
This was first published in March 2004