In order to use ntldr multi-boot you must do 3 things
(1) Have a working Linux with its boot loader availabe for copying (100% complete in your case)
(2) Copying first 512 bytes of the bootloader and make it available in the "C" directory of XP
(3) Amend boot.ini to include the Linux
You should try it out to complete your understanding of both boot loaders.
Lastly, Lilo is also simple to play with. In terms of power anf easy to work with it falls mid way between Grub and ntldr.
In chainloading one boots another boot-loader instead of the system itself. The system is booted by its own boot-loader. Therefore you can keep every boot loader of the systems you install and can choose any of the boot loaders into the MBR.
Glad that you now got the bull by its horn.
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In case you run into difficulty of getting ntldr working have a loook at #23 post of
this thread