I went to the slackware website, and went to the "install help" link:
(
http://www.slackware.com/install/bootdisk.php)
From there I went to the "Selecting a Boot Disk" link:
http://www.slackware.com/install/
There I saw this at the top of the page:
"In order to install Slackware Linux you must boot a small version of it from diskette."
Sounds to me like you don't install Slackware by booting from the CD, but from a pair of boot floppies.