Hey I know you probably already know this but the slackware 9.0 iso doesnt come with the kernel headers for the kernel. You need to download them from your favorite mirror and install them. If you have compiled your own kernel i.e. 2.4.20 or 2.4.21 make sure you have a symlink in /usr/src from linux-2.4.20 to linux. The nvidia drivers look for /usr/src/linux when looking for the header files.
Hope this helps a little