Some older computers fail to boot the Slackware CD or DVD.
You can try one of the alternative installation options: PXE install, booting from a Slackware bootable USB drive (also see http://alien.slackbook.org/blog/inst...b-thumb-drive/
You could also try burning my mini ISO image to a CDROM (see http://www.slackware.com/~alien/slackboot/mini/13.37/
) because that ISO is created a bit differently so that it boots on computers where the official DVD will not boot. You will then have to install Slackware packages from an internet mirror (FTP or HTTP installation option).
Or even, if you have a second computer which does boot the Slackware DVD, you can use that second computer as a temporary PXE server (it is out-of-the-box so it will not be difficult at all) after which you can PXE-boot the desktop and install Slackware from the DVD in the PXE-server computer. See http://docs.slackware.com/slackwarexe_install