First of all, don't do it.
Second of all, if you do, make sure you pick 'expert install' - type 'expert' at the boot prompt or whatever. As far as I can tell, it tried to install grub automatically and barfed - which is good because I'd have been extremely pissed if it had succeeded. Then when you pick expert, you'll have to answer a lot of stupid questions and still not get some of the basic stuff you'd like but you'll at least be able to duck the boot loader install. Then just set up Slack to boot it.
It's a dumbass installer - it glues the partitioning, formatting, and fstab assignment together in a really stupid way. I *had* my partitions and they *were* formatted - lemme write my frigging fstab and dump packages on them.
Also, make sure you create your partitions using Slack's fdisk - I have *no* idea what the hell the installer tried to do but it created freaking *gaps* between every partition it made when I let it do it itself the first time. Had to redo it myself.
People complain about Slack's installer. Pff.
Anyway - good luck!