This is what I did to get Snow Leopard and XP to dual boot with NBI. I have a 10v, 160GB HD. I used an external DVD drive to install XP, but USB for Snow Leopard install.
1) Install Snow Leopard using this
2) When Snow Leopard is booting fine, go to Disk Utility -> Partition and add a Volume. Click on the new partition and select MS-DOS (FAT) from the Format list. Click Apply.
3) Boot to the XP setup DVD. Install XP to the FAT partition using the NTFS quick format. Install the hardware drivers needed for XP. After this install XP becomes the primary boot.
4) Boot back to the OS X using the Snow Leopard DVD USB. i.e boot to USB, when the timer is running, press the ESC button and select your harddrive's OS X partition.
5) When in Snow Leopard, run NBI again, just in case any clean up in needed. I don't know if it is. I also check the 'Generate a system specific dsdt.aml file'.
6) Open terminal and type the following from the first post in this thread:
sudo –s (to allow the proper access)
(enter admin password)
fdisk –e /dev/rdisk0
p (should show both partitions)
f 2 (sets the second partition to active for booting)
w (write the changes)
q (quit the fdisk application)
That's it! Thanks for the additions to this thread to make this a simple process.