The problem with installing XP on modern computers is that XP doesn't recognize SATA drives and running a Windows XP install disk will give you the message "Windows XP was unable to find any Hard disks".
You need to create a new installation disk containing the drivers -- if you visit this site and go through it the process is explained.
You'll need to get rid of the Windows 7 MBR on your HDD as well as the boot process is different.
Look at the various bits of documentation and I think there is a tutorial on the Driverpacks forum.
This is the site. All DriverPacks | DriverPacks.net
It's been a LONG time since I "physically" installed XP although I have created Virtual Machines where the problem doesn't arrive.