You need to make free space for Linux. If your Windows is running NTFS, you need Windows tools (there's nothing that can do this on Slackware cd). You need to resize the partition - you need Partining Magic or a WIndows utility (before you start resizing run defrag & scandisk).
Then you need to decide which method of boot after install you prefer:
1) You can boot directly from Linux bootloader (will be installed in MBR, may be deleted during Windows reinstall)
2) Boot from XP bootloader (requires changes in XP boot.ini, not hard, but you must be careful)
Which one you choose (you need to know before installation)?