Here is the easiest way to do it:
Put in your original XP cdrom (manufactorer or Microsoft
disk), reboot, then go into the bios and change the boot
order to first boot from cd, then save and restart
computer with cd in you will get a prompt that
states "boot from cd, press any key" press any key to
envoke the boot, you will see the cd start up and start
reinstalling, you will get a prompt to repartition first
then reformat, make sure you delete the partition and
remake it then reformat. If you don't know this much you
really should take it to a professional. Good luck!