It is possible that something is wrong with your hard drive, the power or signal connections to it, or with the disk controller on the motherboard.
First, power off and check all connections. Then try re-installing XP.
If XP still fails to see the disks, think about whether they are plain IDE (ATA) disks, or are thye "RAID" or "SCSI" or maybe "SATA". If any of these, XP will not see them unless you hit F6 early in the install process and provide the appropriate drivers on a floppy (not on a CD, only on floppy). These drivers come from whomever made the disk controller, usually the motherboard maker.
If none of these work, then check whether the hard drive is "seen" by the BIOS. Watch the black screen with white characters that appears just as the PC reboots, long before it reads the XP CDROM. In order to see everything, watch for a message about pressing some pecal key(s) to view the "POST". Do not press the key(s) to enter the BIOS setup, which may also e shown briefly on the screen. If the BIOS can not see the disk, then XP has no chance. Think about removing the disk and testing it in a different PC.