You will need at least 4 partitions:
1. An NTFS partition for Windows XP (linux can read this but not write)
2. A FAT partition to share between the two operating systems. (both Linux and windows can read and write)
3. A small SWAP partition (a few hundred megabytes to be used like virtual memory in Linux)
4. A Linux partition formatted as REISERFS or EXT3.
The swap and linux partitions can be made by the linux installer.
My first suggestion would be to get Disk Director from www.acronis.com
and RESIZE your existing NTFS partition so that you don't need to re-install Windows, create a FAT partition, leave the rest as free space and install linux in the free space.
My second suggestion would be to attempt to do the same thing with the Mandrake installer if you can't afford to purchase Disk Director. It is supposed to have the ability to resize NTFS partitions. Mandrake is much better for starting out with Linux as well. You are likely to get frustrated using Slackware if you have no experience unless you REALLY want to read manuals and learn. Mandrake is a lot more point and click.