Not sure about the bootdisc in linux, but probably the easiest way to do what u want is to go ahead and install XP and if lilo doesnt work use the slack cd1 to reinstall lilo so you can boot both systems
Run setup and just choose your target partitions (dont format obviously
) then skipping eveything else go to configure and skip all cept lilo setup and saving the new fstab file.
Once lilo is installed skip everything else, exit setup and reboot