it looks like the framebuffer device doesn't like your graphics card. When selecting "linux", type this after the "boot: linux" prompt:
, or vga=normal
, or vga=high
. This opens a standard vga console, instead of the cool 1024x786 console with the penguin logo.
After loggin in, you could try to run "startx". However, I'm affraid you'll get the same black screen. This thing could be solved by installing the slackware kernel sources, and installing the geforce kernel modules. ...and off course, telling X it should use this driver. perhaps the installer some tool for this, or you need to edit /etc/X11/XF86Config manually. (not a great thing if you don't know where to start)
Perhaps you could try the newer slackware 9.1 version too...