Well the best way to go about it is to boot up a Live CD and use the terminal program cfdisk to create
(1) hda1 (or sda1 for Sata) for XP, make the partition type 7 for ntfs filing system.
(2) hda2 and hda3 for Slackware. Partitions created by Linux automatically Type 83 suitable for any Linux installation but you need to make one of the two Type 82 for swap. For example say hda3 is swap.
When you install XP it will know where to go immediately.
When you install Slackware just tell the installer to install Linux in hda2 and everything will fall into place.
Lilo is the standard boot loader in Slackware. It should dual boot XP automatically after the installation. If it doesn´t edit /etc/lilo.conf to make sure it has the following lines inside