Go into BIOS by pressing (F2 or whatever key your system accepts for entering Bios) and select the boot options.
Make CDROM/DVDROM as first boot device.
Make Hard disk as second boot device.
Ignore other boot devices. Save settings while you quit the BIOS.
Insert the XP installation disk and reboot the system.
If the system comes with a prompt on screen, "Press any key to boot from CDROM" means you already have some partitions with windows installed.
Press any key to boot with CDROM.
If you do not have any partitions with windows installed then the Windows XP setup program will run automatically from CDROM.
From here on follow the below tutorial for clear and precise instruction to fresh install windows XP on your system. Step By Step Guide To Clean Installation of XP