I used cfdisk from the command line when installing slack 10. I am a noob and the program is pretty self explanatory as long as you know what you are doing. I would read up on partitioning if you are unsure.
Know what size you want your :
-root partition ( the amount of space you are allocating to slack.) ( space to install slack to plus room for the stuff you want. apps ,.mp3, .jpg .avi etc.)
-swap (I have been told many times to make swap 2x as big as the amount of ram. (ie. if you have 256 do swap 512mb.) I think this is debateable.
-home (if you are putting /usr/local on seperate partition.)
-ntfs partition. (WIN)
are you going to use ext2-3or rienserf(sp?)?
what boot loader you using lilo or grub?
are you doing a full install of slack?
Are you sure all your hardware is compatible with linux?
I am just trying to give you the things to think about that snuck up on me during my first couple of Linux installs.
I haven't done a dual boot with win and slack.
I would suggest backing up everything you value off win.
What's the worst thing that can happen. You mess up and waste your win partition and make a mess of your hard drive. I messed my amd comp with a fedora core install on a xp box. But I learned a lot from it.