Boot your computer from your WIN XP cdrom and follow all
prompts. You will be given an opportunity to install XP
in a partition on your existing HD. You can choose to
delete the existing drive partition and then create a new
partition for installing a new copy of XP.
Obviously you'll erase everything on the HD when you do
this.
Consider buying a larger drive and using the transfer
utility in XP to move the data from the old drive to the
new one.