Boot from the XP installation disk and start the install process.
When it comes to choosing a partition, make sure you select the one on the new hard drive
Basically you continue through as if you were doing a clean install on any hard drive.
Create Boot Loader
Once installation of XP is successful you can now go through and install the latest Microsoft Updates and drivers.
You will find that the machine is booting directly into XP at this time. This is due to XP writing it’s bootloader over Windows 7’s. To get both XP and Windows 7 as an option at the boot screen you can use the free utility EasyBCD.
When you open EASYBCD with the Reinstall windows Vista/7 bootloader selected . click on Write MBR button
Reboot the PC
After getting the bootloader back you should see both XP and Windows 7 as options in the Windows Boot Manager.