If you can, I've used this trick before with some success without an optical disk drive...

1. Format a 10~15GB partition with FAT32.

2. Download and install the Cygwin base system.

3. Download and run AlienBOB's mirror-slackware-current.sh and allow Cygwin to sync all the files and download everything (the iso creation is optional but unneeded).

4. Copy the entire downloaded archive to the new partition and use the USBIMG2DISK.sh utility in /usb-and-pxe-installer directory and create a USB install bootstick (use Cygwin if needed).

5. reboot and have your BIOS boot the USB stick, mount the drive after logging in to /mnt/install (you may have to create the directory and mount it), and when asked for your installation location, give it the mount point and all should be well.