(slightly edited for 13.37)
First, you will need the Slackware install cd for whatever version of Slackware
that you have installed. Insert that cd into the drive and reboot the system
(this will probably have to be forced with a manual power off).
When the install cd boots, you will notice some instructions for booting into
your system - ignore those. Instead, let the installer boot (as if you were
going to do a fresh installation).
Note: you will probably have better results if you boot the huge26.s kernel
on a Slackware 11.0 cd, or huge.s or hugesmp.s on later Slackware
Once the installer has booted, login as root (no password is needed) and
mount your system's root partition to /mnt
For example, if your system's root partition is /dev/sda1, then do this:
mount /dev/sda1 /mnt
Following the above will get you access to your file system.
To access the file system as your root filesystem, do 'chroot /mnt'
The other part of the question is how to recover.
1) If you want to uninstall the nVidia proprietary driver, then it will depend on how you did the initial install.
If you used a SlackBuild and made a package, then you can simply use 'removepkg'.
If you used the nVidia installer, then run the installer with the '--uninstall' flag.
2) It may also be that you simply need to specify a kernel boot option so that you can boot successfully.
You could try editing /etc/lilo.conf and adding ' video=800x600' to the append= line.
Type 'exit' to exit the chroot.
Type '/mnt/sbin/lilo -r /mnt' to rerun lilo.
Remove the CD and hit Ctrl-Alt-Del to reboot.
The installer said that the nouveau driver was incompatible and did something to remove it from the xorg.conf file.
The nVidia installer blacklists the nouveau kernel driver from loading by making an addition in the /etc/modprobe.d/ directory. It will also build an xorg.conf file if you request that.