Hi
why get rid of W7 its a lot better than XP and more future proof, XP is good dont get me wrong, but its now an old OS and wont be supported in a few years by MS.
However if you really want to
set your BIOS to boot from CDROM, if your laptop doesnt have one do a google on IDE to USB adaptor, insert your XP disk and boot from it, dont choose to repair choose to delete the current install and then just install XP.
Its quite straight forward, but you will need to make sure you have all the drivers for it, such as video sound, network etc etc before you start.
doing a google on installing xp will give loads of results.