nVidia's code is not open source and is, therefore, not licensed under the GPL. Linux doesn't like this, and chances are, you're seeing a few 'tainted' warnings alongside a number of other errors [not attributed to the non-GPL code]:
# What does it mean for a module to be tainted?
* (REG, contributed by John Levon) Some vendors distribute binary modules (i.e. modules without available source code under a free software license). As the source is not freely available, any bugs uncovered whilst such modules are loaded cannot be investigated by the kernel hackers. All problems discovered whilst such a module is loaded must be reported to the vendor of that module, not the Linux kernel hackers and the linux-kernel mailing list. The tainting scheme is used to identify bug reports from kernels with binary modules loaded: such kernels are marked as "tainted" by means of the MODULE_LICENSE tag. If a module is loaded that does not specify an approved license, the kernel is marked as tainted. The canonical list of approved license strings is in linux/include/linux/module.h.
"oops" reports marked as tainted are of no use to the kernel developers and will be ignored. A warning is output when such a module is loaded. Note that you may come across module source that is under a compatible license, but does not have a suitable MODULE_LICENSE tag. If you see a warning from modprobe or insmod for a module under a compatible license, please report this bug to the maintainers of the module, so that they can add the necessary tag.
* (KO) If a symbol has been exported with EXPORT_SYMBOL_GPL then it appears as unresolved for modules that do not have a GPL compatible MODULE_LICENSE string, and prints a warning. A module can also taint the kernel if you do a forced load. This bypasses the kernel/module verification checks and the result is undefined, when it breaks you get to keep the pieces.
* (KO) According to Alan Cox, a license of "BSD without advertisement clause" is not a suitable free software license. This license type allows binary only modules without source code. Any modules in the kernel tarball with this license should really be "Dual BSD/GPL".
As regards the errors you might be receiving during the boot-up process, the only thing I can imagine is that you're trying to load certain modules that conflict with other modules already loaded. For instance, if you enabled ALSA
[Advanced Linux Sound Architecture] during the setup process when you were installing Slackware, then you do not need to use the nvaudio driver, since ALSA supports the onboard nForce APU. There are possibly other causes of the errors you are receiving during the boot up process, but this example seems to be pretty common amongst those using nVidia's drivers. If you don't know whether the ALSA drivers are the problem, then you can issue an 'lsmod' at the commandline to check what modules are currently loaded. If you see modules that look like snd_xxxxx, then they belong to ALSA, and you have no need for nvaudio.
Now, regarding the Linux crashes you are experiencing, where Linux completely freezes and becomes entirely nonresponsive, if you are using the nvnet driver for Ethernet communications, then this is likely the cause of the freezes. There have been a vast number of problems with this driver, and there have been many pleas to open source the code in hopes that the Linux hacking community could improve upon its deficiences without waiting for nVidia. Instead of waiting for nVidia, a group has reverse engineered the nvnet driver and released it. It is still in the alpha stage, but there have been many good things said about it. It's called forcedeth [google it], and it is a patch to the 2.6.x kernel. But, it isn't a must, although I recommend it.
Finally, overall, I simply recommend downloading and building yourself a nice new 2.6.x kernel. It comes with some nice new features, and the problems are, by far, considerably less. Of course, it will require you to get a little messy with some kernel configuration, but this is what Linux is all about.
Check out some things that have been said in another forum: