Ok, that's probably a good place to start then. It's pretty basic, so let's go with it.
Let me give you some general info, and then try to use it in an example:
Whatever partition /boot is on is important. If it's on the same partition as everything else 'arch' (/dev/hda9) that's fine, just remember that when you compile and copy new kernels to your 'boot' directory. For multiple distro booting I tend to desire a seperate partition JUST for kernels (/boot). This way, when I boot any distro I can just mount /boot seperately and have every kernel there just as it would be an any other distro I boot.
Regardless, you need ALL kernels in a single directory (there are ways around this, but take this at face value for now). So, copy your slackware kernel to your arch's /boot directory. Once this is done, you'll need boot entries for these kernels. For ease of explaination I'll just call your kernels by your distro's name, you can name then WHATEVER you want, even pedro-1 and pedro-2 if that seems appropriate.
So, now we have these kernels in /boot in arch:
ls -lh /boot:
And we might also have initrd's:
These ARE NOT necessary, and if they aren't there, no sweat, just move on.
Now, edit /etc/lilo.conf within ARCH to have some entries that look similar to this (append to the existing LILO so you don't have to make any changes, just additions):
image = /boot/slackware-kernel
initrd = /boot/slackware-initrd #remember this doesn't have to exist, leave it out if it doesn't
root = /dev/hda7
other = /dev/hda5
label = WinXP
table = /dev/hda
other = /dev/hda6
label = Win2k
table = /dev/hda
This is just generic info, tailor the /dev's to your situation.