If you can boot into slack, run cfdisk, and if that fails run "fdisk /dev/hda" (Assuming that's your HD)
Once in there simple type m for a list of commands and you'll see the command to delete a partiton, delete 1-howevermanyyouhave and type I think "W" to write then "Q" to exit.
Once that's done go into cfdisk and it should work now and make the partitions you want (Remember makesure one type is Linux and another type is Linux Swap (Swap is type 82))
Now once that is done go to write and write to the partition table then exit.
You may now (unless setup tells you to restart), run slackware installation.
Start at the screen to setup swap and everything else is for the most part automated.
When you come to the screen to format your HD choose ext2 (or if you want, ext3). I've had some bad history with ReiserFS so I wouldn't recommend it (IMHO). Now after your drive is formatted it should try to automount (hopefully it will work, if not run the steps again and choose manually mount it).
NOW, you are at the package screen. All of Linux and it's programs are availible in packages. You're going to want a lot of them, probably all of them.
Once you are done selection you can hit Expert, or any of the other options to choose which PROGRAMS inside the package you want (nifty eh?). After that you should have a full install.
Now, after the install is done it should automaticly run the things like searching for a modem, setting screen fonts, setting up network, etc.. which may be different issues we can fix here but nothing should go wrong. You will I think in the end get a LILO (LInux LOader) screen. This is what boots Linux. LILO (iirc) follows the Multiboot standard (also used by GNU GRUB) which means that after you install Windows, you can run "liloconfig" again and reinstall Lilo with Windows installed.
Now, LILO may fail on install and if it does, install it on the MBR (I don't think this is bad since afaik if you ever decide to install Windows alone and it (or it may be the linux partitions not being present) uninstalls LILO)
After that a simple "reboot" will reboot your system. Popout the CD and hope to hell it boots into LILO (red screen
(Note: I did not explain using the second CD to grab KDE, Gnome, and other things).
Happy to help,
Nelson
PS: Any corrections or improvements are welcome.