If the old XP-64 uses the same mbr as the new XP-64, you should be able to create/edit boot.ini now to add the old one to your boot menu.
Something like:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Microsoft Windows XP-64 New" /fastdetect
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Microsoft Windows XP-64 Old" /fastdetect
The disk/partition numbers have to match your setup, of course.
And WINNT is whatever your Windows directory is. | 
