First of all you need to make sure whether Windows 8 can boot with Secure boot disabled. If so, then Supposing the system uses the UEFI partition for booting, all you should need is installing elilo (EFI-enabled LILO), which is shipped with Slackware. All it does is copying kernel to the EFI boot partition.
If for some reason you need to use Secure boot, you either have to use the signed shim that loads GRUB (which in turn loads the kernel) or sign your kernel yourself and load the key into the UEFI (this usually is possible, but not widely used for obvious reasons).
In any case it might be a good idea to make at least partial backup of the HDD contents (ideally on device level).
As for booting without CD: if you happen to have another computer at hand, booting over network is usually not too difficult to set up - you just need a basic DHCP and TFTP server, e.g. dnsmasq (which is packaged in the Slackware tree; and there is some documentation on how to do it as well).
Another option is of course taking the HDD out, putting it into a machine with DVD, installing whatever you need and putting it back. It would also make it much easier to backup the drive.
Back to the problem: if you already installed Slackware, are just unable to boot into it yet you can boot some linux (from USB or network, even the Slackware install image) on the machine, just do so, mount the Slackware partition somewhere, bind mount the important stuff from the running linux there, chroot into it and do all the required things. Basically you need something along these lines:
mkdir /slackware
mount /dev/path/to/installed/slack /slackware
for m in dev dev/pts proc sys; do
mount --bind /$m /slackware/$m
done
chroot /slackware
# optionally mount partition with your kernel,
# so that elilo can find it
# edit /etc/elilo.conf to your liking
elilo
exit
for m in sys proc dev/pts dev; do
umount /slackware/$m
done
umount /slackware
reboot
