How do I modify my XP's bootloader to add my Linux install?

I would not expect Boot.ini to cooperate in that regard.
Instead I would make a lilo boot floppy to load Linux. It can reference files on your secondary disk directly or can be self-contained. There's plenty of room in a 1.44Mb floopy for a configured kernel and initrd - the modules themselves will come off the HD. I've appended my lilo.floppy.conf as an example (no initrd)/
BTW you have a good philosophy to not touch your main HD - at work I used to dual boot with lilo and with linux on hdb, until hdb had a head crash. Then the lilo boot would not load Windows because the map file was on hdb. So now I boot windows off a lilo floppy which has an "other" entry and nothing else :/

# LILO configuration file
#
# Start LILO global section
boot = /dev/fd0
map = /fd0boot/map
prompt
timeout = 1200
read-only
root = current
compact
geometric

# End LILO global section
# Linux bootable partition config begins
image = /fd0boot/v2.6.7

Entry in /etc/fstab:
/dev/fd0         /fd0boot         ext2        noauto,user      0   0

The boot floppy is ext2 so msdos filesystem can be a module

you can do it with liloconfig

also you can add this to your lilo.conf

other = /dev/hda1
     
       label = Windows
       table = /dev/hda

if your hard disk if on your firs chanel and is master it's HDA
and type
lilo

install the lilo to the partition where yoru linux is then edit the boot.ini and add the line like this
multi(X)disk(X)rdisk(X)partition(X)\WINNT="LINUX" changing the X according yoru configuration

Mkay, how do you install LiLo?

I'm a complete newbie at Linux. I can boot to the recovery using the DVD I burned, but have no idea from there what to do.