Descartes' Error is the term used by materialist (or monist) philosophers to describe Cartesian Dualism -- the idea that Mind and Body are separate things, made of separate substances. More on this in a bit.
Animals (and it's amazing how many people don't understand or believe that we're animals) don't have hardware and software (or Body and Soul, as most European religious traditions -- and Descartes -- would have it). Instead, we have what the cyberpunks of the early 1980s called "wetware". I'd rather call it "meatware" -- that has the added benefit of pissing off PETA.
Meatware is not like hardware and software: when you unplug a computer, hardware stays the same (except for transient voltages), and software just fades away (going back to being unactualized ones and zeroes on the disk, or disappearing entirely if there's no copy on disk). In contrast, if you "unplug" meatware, it just rots.
Meatware is hardware that changes, embodying the software in its very being. Ironically, this is a lot like some of the original computers, which were hand-wired to program them; the programs were not separate from hardware at that point. It was a conceptual great leap forward to separate hardware from software (see punch cards, Jacquard looms, etc.), but it definitely also meant that Descartes was right -- but about computers, if not human brains.
Discursive aside: I am a materialist. Not in the sense of liking to acquire worldly goods (though I'm pretty happy to do that too), but in the sense that I believe there is one kind of Stuff in the Universe: Stuff. Matter and energy (which are both Stuff). There may also be Dark Matter (which might be anything from supersymmetric partner particles to other exotic stuff we haven't figured out yet) and possibly Dark Energy (which could also be a supersymmetric partner particle, or might just be the Cosmological Constant), but it's all Stuff. It's not Souls. There is no Soul-stuff, separate from Matter-stuff. That's the point of philosophical materialism.
Descartes' Error is believing that there is Soul-stuff separate from Matter-stuff. It can be easily demonstrated to be an error, like this: Soul stuff either affects Matter, or it doesn't. If it does not affect matter, it has to be entirely separate, and then Soul just becomes some metaphysical attribute that cannot be talked about in any empirical way, because it cannot affect or interact with Matter. If Soul does interact with Matter, it's just another kind of Matter, and is in the realm of empirical science.
Discursive aside to the discursive aside: this does not mean that I believe that morality (or ethics, or whatever) comes down to physics. I simply don't, and anyone who makes the claim that philosophical materialists are undermining morality because we deny the "Stuff-ness" of Soul or Spirit is simply talking trash. Just as Chemistry deals with a regime of behavior of Matter that fundamental Physics doesn't, and Biology another regime of Matter Behavior, I believe that philosophy, law, politics, morals, ethics, and other ways of describing the world that deal with Human Behavior in Relation to Other Humans are disciplines that deal with Matter Behaving in yet another regime.
So: "nature vs. nurture": it's not just brains. It's the entire life of each organism, from DNA up to the organism level, including all its interactions with the physical world, including other organisms.
What any organism is now, at this instant, is a deeply historical integration of many many things: its genes have been interacting with its environment for its entire life. Its environment may have included being incubated in its mother's body (if it's a mammal, for instance) and being washed in hormones from itself and its mother (and potentially from siblings from the same litter) for the period of its gestation. There is diet (or soil fertility and rain for plants). There is exercise (for animals). There's stress level. There are accidental things: cosmic rays, viral damage, physical damage. And so on.
ALL of these contribute. All of these affect the others. Genes are turned on or off: genes activate and deactivate depending on environment, and the rates at which they are activated and deactivated change depending on environment. Animals have equisitely complex systems for adapting to their environment. Everything interacts.
Are the Pima Indians who live in Texas and eat American fast food, and many of whom are hugely obese, "naturally" fat? Their cousins in Mexico, who eat a basically Aztec diet, are skinny. Are they "naturally" skinny? Or is it all "nurture", and diet? Well, some people eat lots of fast food and don't gain weight.
Is Yao Ming "naturally" 7'5" tall? Might he have been taller with a different diet? He certainly could have been shorter with less protein at the right times in his life.
Lots of people (including me before I started to learn about it) think that genes are like a computer program -- a batch computer program, that runs once, straight through to completion, and then is done, like following a blueprint and then your building is built. It turns out genes are a lot more like computer programs than we guessed -- like modern computer programs that interact with the world: operating systems, or phone switches, or complicated databases. The code is constantly running, but different parts are active at different times. Genes are turned on and off, or have their activity modulated, based on what's going on in the cell. Hormones influence gene activity. Levels of proteins influence gene activity. Other genes' activity influences gene activity. It's bloody complex, and it's going on all the time. There's no "completion" of the genetic program. There's no done.
Like many other things in the world, the false dichotomy of "nature" vs. "nurture" is bogus, because the world is a system -- a complex, interlocking, feeding-back system that doesn't have neat boundaries. Deal with it.
Anyone who says "nature vs. nurture", and believes it, is sadly deluded.
Here endeth the rant.
Edited to add: go read Pharyngula today for a neat take on genes and development.