In Disk Mgmt, rightclick>delete the Logical Partition you created for XP, then rightclick again to Delete the Logical volume if it remains as shown by the green box below. Partition or Volume - Delete
Now boot the XP CD to install, see if it detects the HD so you can create a partition in the deleted space. Be sure not to touch the other partitions!
If not create a fourth primary partition there using free Partition Wizard bootable CD following Option One here: Partition Wizard : Use the Bootable CD
Try install again, if necessary insert SATA controller drivers if the HD is not detected on screen shown just above. SATA Drivers - Load in Windows XP Setup on Dual Boot
If this is too much trouble, consider virtualizing XP using XP Mode in Ultimate or Pro versions, or free Virtual Box, Virtual Machine or VMWare.