Hi! Welcome to LQ!
So, you're trying to dual-boot Slack with XP Pro. Ok...
First of all, how much RAM do you have? If you have somewhere in the midst of 512MB-1GB+, then I usually don't put a swap, although I do for smaller ones. Some people go on 'swap size = half of RAM size', but I go on a swap=RAM basis. It's up to you, ultimately, whether you want/need a swap space or not, and how big it should be.
As for the partitions, I could help you with that myself if you would like. Yet, if you would prefer a proper tutorial, there are hundreds waiting to be found by you on Google. A simple "dual-booting slackware and windows" will turn up more results than you need!
From your post, it looks like you've only had a little bit of experience with Linux...are you sure you want to try Slackware so soon? I agree that it makes for a steeper learning curve, but sometimes you might get stumped badly and get fed up with it - just a small point!
As for the partitions - I'm guessing XP currently takes up all you HD space at the moment? I use Partition Magic to resize my partitions (it has never failed me), although there are tons of freeware ones for Linux too (which are also very good). Resize it to half its size, and then create ONE Linux partition, with mount point set to "/", and file system "ext3" (or another if you want). Yuo may want to have a separate /home partition, but again, that's up to you.
HIH,
J_K9
edit: Check this page out for resizing partitions:
Resize FAT32 and NTFS. There's also anotehr method in my Public Bookmarks...