If you already have suse installed, your disk is probably partitioned in tree or four. (XP, Suse, swap and perhaps /home). Hopefully /home is on a separate partition so you don't have to backup your home folder. All this is speculations since I haven't seen the actual partition table and /etc/fstab file.
The slackware installation are pretty self explaining if you have some knowledge of GNU/linux. Slackware as default comes with lilo (And grub in extra). I can't remember if you can skip overwriting MBR. I have always installed it. Anyway, I'm guessing you might have problems with automatically detecting XP. (That's just a guess, since I don't have XP/windows on my computer). If you're planning to install grub again, I would backup the old /boot/grub/menu.1st from suse, and modify it to suite slackware. It might not work, but it's worth a try.
Other then that, there isn't much to it. After you start your slackware CD, you might want to verify your partitions so you don't install slackware on the wrong partition. Check with fdisk/cfdisk and mount them to a temporary folder to check it's content. If you don't have a separate /home partition, I would split the partition used for suse. (Remember to backup).
Before the actual installation, it will ask you what to install, I suggest a full installation, then you wont be missing anything. You can remove packages afterwards, and learn how to do a custom installation later.
Another file you could backup is /etc/X11/xorg.conf, since it most likely is already configured for your videocard. There are probably more things thats handy to copy from suse, but it doesn't really matter. Slackware comes with a great standard installation
If you have any other questions, the slackware forum in here is really great