Go ahead and format the drive you have windows 7 on to a ntfs file system and unplugg it. Then make your XP HDD your primary boot device in your bios.
Use your XP install disc to repair the MBR (or maybe you don't have to. Just unplug the drive with win 7 and make your XP primary boot. If it boots to XP then you don't have to do this step)
Boot with the XP installation CD.
When prompted, press R
to repair a Windows XP installation.
If repairing a host with multiple operating systems, select the appropriate one (XP) from the menu. If you have only one operating system, enter 1 to select it.
Enter the administrator password if prompted.
To fix the MBR, use the following command:
and ENTER to fix the MBR.
then type exit
Reboot and make sure you can boot to XP.
If you can then plug the second drive back in and load windows 7 on it. Leave your bios settings alone, use this as secondary drive. Win 7 should write the bootloader during install.