If you have Ubuntu running currently there is no problem dual booting with Slackware.
1. Install Slackware as usual. When the dialogue window asks whether Lilo boatloader be installed or not just click "No" and proceed to remaining steps without installing a boot loader for Slackware.
2. Boot to your Ubuntu and edit its /boot/grub/menu.lst; under the the line ## ## End of Default Options ## are listed the pointers to boot up, there in the beginning line enter the following:
THIS IS ONLY AN EXAMPLE, CHANGE SPECIFICATIONS ACCORDING TO YOUR OWN SITUATION:
## ## End Default Options ##
title Slackware 12.2 GNU/Linux
kernel /boot/vmlinuz root=/dev/sda4 ro quiet splash
If your slackware is installed in the second partition you may change the 'root' line into
and then the kernel line must also implement the same, e.g.
kernel /boot/vmlinuz root=/dev/sda2 ro quiet splash
Under Slackware the initrd is not built upon installation, you have the option to build it yourself later <it is not a critical option> for systems having good RAM sizes.
Usual problem for any newbie editing /boot/grub/menu.lst lies in NOT ACCURATELY POINTING the target kernel. Don't be surprised if you can't hit the target right away: everybody falls the first time. Just be sure you have read enough about difference between Grub syntax of using "hd0,0" instead of Linux syntax of "/dev/sda1" when technically they point to THE SAME TARGET. Refresh your Grub first, here's a good tutorial:
Don't forget to read first the Slackware book in your cd /slackbook/slackbook.pdf
Or visit Slackware Essentials:
Check back if you need help how to build an initrd later if you decide.
BY THE WAY:
What if your Grub in Ubuntu was overwritten by Lilo?
Solution: Reinstall Grub booting from Ubuntu CD. when it is installed configure /boot/grub/menu.lst accordingly as above. This if you don't know how to configure Lilo. But I always advise others to use Grub.
Or you can install Grub from the Slackware CD it located in the cd/extra/grub.
Hope this helps.