You will need to format OUTSIDE of the OS. Using your XP disc as izzo said above.
steps you should do.
BACKUP EVERYTHING. You will lose everything unless you back it up and get it off the hard drive.
Boot off your XP disc as mentioned above. (put disc in, reboot to bios, make sure your CD is the first to be checked at boot, save and exit if you had to change it, wait for prompt to press a key to boot from cd and hit a key to do so. [sometimes it will boot the cd directly without prompting you])
Once in the XP setup, if you want a 100% Clean pure install. when it asks you what partition to install XP to, Look at the bottom for the key to hit to delete partitions, and Delete the one with your XP on it [will probally only be one on most systems]. (Once you delete the partition the data is GONE, so before hitting that delete confirm button, will be your last chance to backup anything you still want on your windows Drive)
Next if you only had one drive before, now is your chance to partition your drive and make a separte volume just for your data on your drive. (Eg. C will be your OS, Apps, and Games. As you will need to reinstall them anyways when you reload your OS. and D: for your personal data, Save Games, That super secret project you've been working on.. etc etc etc..) that way when you wipe your OS again.. you wont lose anything, and its easy to backup an entire drive, just image it
Your next option will be a format option, dont do the quick method, a full format may take longer but is just another step to guarantee you've got a fully clean drive ready for your new OS.
Just remember once your OS is up and running. Install your Anti-virus, firewall, and run all the updates you can find. Get your latest drivers, and establish a Baseline.. once you know how your system runs on a pure clean install. make an Image of the drive, (Norton Ghost is a great tool to have, but there are alot of other ones out there, just google Disk Cloning tool, or Disk Imaging tool.)
Enjoy your smooth new ride..
/ system b 