OK don't worry its not as hard as it sounds, first if you havent already, go grab the kernel source from any Slackware mirror, ( assuming you don't use your own kernel) should be inside ~/slackware/k/ directory, from memory I think slackware 9 uses kernel 2.4.20 out of the box.
It should be a .tgz file, install it by using "installpkg" command.
after that's done go to /usr/src/ , you should see a symbolic link "linux"
now grab the nvidia nforce driver source (the .tar.gz one)
extract it by doing
tar xvzf NVIDIA_nforce-1.0-0261.tar.gz
it should create a directory called "nforce"
cd in there, su, then do
(generally when compiling and installing from sources you need to first do a ./configure , but you don't in this case for the nforce driver)
The "make" command compiles the source code into binary, the "make install" copies the binaries into places where they should be installed.
Lastly edit your /etc/modules.conf then add
alias eth0 nvnet
alias sound-slot-0 nvaudio
alias usb-interface usb-ohci
So that the modules gets loaded on boot, do a "modprobe nvnet" if you want to load it straight away.
note if you only want the NIC you might want to leave out the other 2.
hope this helps.