As you already have Ubuntu on it you probably have grub on it, try this...
on the root of your ubuntu partition mkdir slack14
copy into this /isolinux/initrd.img and the contents of /kernels/hugesmp.s/ from your favourite slackware mirror
reboot
at the grub menu select your normal ubuntu but hit "e" not <enter>
replace
linux /boot/blahblah
with
linux /slack14/bzImage load_ramdisk=1 rw SLACK_KERNEL=hugesmp.s
and
initrd /boot/blahblah
with
initrd /slack14/initrd.img
hit <ctrl>-x
enjoy