XP booting from external box

Ok guys, I got TinyXP installed in my external HDD (which is in an external box that plugs into the pc via usb).
Thing is, I can't seem to configure boot.ini to make the pc boot from my external drive.
Can you guys help?

>>2
External: It's an enclosure where you put your regular IDE HDD and it connects to your pc with an USB cable. Simple enough for your understanding?