I had a similar issue with a much more recent system using SalixOS 13.37 which uses the same kernel and X.org server as Slackware 13.37. The system in question has a NVIDIA GeForce 6150SE chip. Salix OS boots to the GUI by default and doesn't offer a different option during installation so at first boot instead of a login screen I had a useless mess. The problem was that the hardware autodetection produced an incorrect configuration for my chipset.
The solution was to open a virtual terminal and add "Option" "ShadowFB" "on" to a file named "50-device.conf" (without the quotes) in /etc/X11/xorg.conf.d. In SalixOS that file had to be created. The file looks like this:
Identifier "Default Device"
Option "ShadowFB" "on"
After restarting X it worked.
The generic solution I give to people who have either very old or very new chipsets which aren't supported by nouveau is to blacklist nouveau and either fall back to the old nv driver or to install the proprietary NVidia driver. Some really old chipsets are no longer supported by the proprietary driver and some of the legacy proprietary drivers don't play nicely with a new kernel and X.org when used with certain chips. In that case nv is the only viable solution.