Yes, a full install of slackware puts apache, mysql, php and an ftp server on your machine. You don't need XAMPP at all.
If you really want to run XAMPP instead of the slackware packages kill the current apache.
cd to /etc/rc.d/
Look for rc.httpd
Shell command:
XAMPP is a nice product. I have it on an XP box and it works great for that enviroment. But it's really superfluous on a Slackware installation.