Enable Boot from CD in the Bios after inserting your XP disk in the CD/DVD-ROM drive and then do a Repair of XP.
Initially you will need to allow XP to Install and then, when you are given a choice of Installing or the Recovery Console, select Install again.
Next time you will have the choice of Repairing XP. Press R to agree.
XP will now be reinstalled on your system without you losing any data, programs, applications, configuration etc providing they are still on the system.
You will need to have your Product Key handy as well as reinstall SP2 and any XP updates issued since the release of SP2 just over a year ago.