First make sure you have backup of all your data and also have all the
softwares that are currently installed on your system as you will need to
install them again.
Then, put your Windows XP CD in your CD Drive and restart your system. Make
sure your system is set to boot from CD, otherwise you can change this
option in the System BIOS settings.
After restarting, the system will confirm if u want to boot from the CD or
disk? Press whatever key it wants you to press for booting from CD.
Then you will see a screen asking you which drive u want to install Windows
XP on and will give you some options to create and delete partitions. Just
follow the prompts and first delete your existing partition and then create
a new one at the same place. When u try to install XP on the new partition,
it will automatically format it and start with the installation. After that,
you just need to follow the prompts from the installation itself, which
should be pretty easy if you do not have to configure network, etc.
For details, you can refer online resources which are available in
abundance - just search on Google.