You need an XP disk, SP3 is best or you will probably get errors about the hard-drive not being found due to SATA driver issues. With a vialid license ofcourse
Get the drivers for your model from HPs website
Install XP by booting off the optical drive, follow prompts.
If you really want to run XP on the system, I suggest you buy a new hard-drive to install XP on, and put aside your Win 7 one for later use or if the XP setup fails, as it may since your hardware may not work with XP.