You guys are approaching this as if the only way for an Arachne to exist would be for it to be a giant spider thing without any modifications to its spider anatomy. This is silly.
We all evolved from very small animals that lived underwater, yet here we are in human form. A much larger stretch than having arthropods growing into big sizes.
Specially because in prehistoric times, we did have arthropods that were quite big. The largest arachnid ever was a ~1 meter long scorpion (who was also kind enough to let me know about him in the comments :)). That's a little over three feet. It had aquatic cousins that could grow up to 2.5 meters long (that's over eight feet), though. Granted, these beasts had their weight supported by water, but look at that... An arthropod larger than a man! Not only that, but we know that there's less oxygen available in water than in air, so how could the square-cube law allow for that?
Well, besides evidences that our atmosphere was more rich in oxygen millions of years ago, there is also the fact that those arthropods had evolved their internal anatomy to allow for those sizes.
Let's go back to our drider, then. Yes, drider. It's easier to spell and more catchy than "arachne".
Imagine if you will a prehistoric tarantula. It lives in a world where there is more oxygen available than Earth right now, because that's how prehistoric Earth was like. Our prehistoric tarantula is under many different evolutionary pressures, and a larger size is conferring it increased rates of survivability and reproduction.
To allow for that, the tarantula starts developing, throughout generations, some unusual characteristics for a spider. For example, a mutation introduces a third body segment to it. This new segment will go in front of the cephalotorax, much like the head of an insect. It will contain the mouth, eyes, chelicera and pedipalps. It will also be filled with extra book lungs. This will allow for more oxygenation.
Over time, as this species of tarantula grows, it will evolve a circulatory system much like a worm's at first, then that of a vertebrate. A fourth, smaller body segment develops in front of the third segment, containing the eyes and chelicerae, but not the pedipalps.
As for the abdomen, it resembles that of a reptile or amphibian more and more, with different organs going through convergent evolution to take the roles of a liver, a spleen, a pancreas and so on. Spiders have excretory organs in their legs, but this species's excretory organs move to the abdomen over time.
Now, spiders are known for being very agile with their legs, and weaving webs with them. Our specific tarantula, though, is growing larger by the millenia, so its six frontmost feet are only used for supporting weight now. Eventually they will be completely flat and fingerless (yes, spiders have fingers - actually microscopic claws, which is how they weave and cling to stuff). Only the last pair of legs will have fingers for weaving.
To make up for that, the pedipalps start evolving into manipulators. They evolve more and more segments until they end up looking like vertebrate arms with hands, again due to convergent evolution.
Two more things are needed for this spider to reach a "drider template":
It will evolve a "neck" between the two frontmost segments, to allow it to rotate its "head". It may have eyes in all directions but the two eyes with the best sight are still fixed, and rotating a head is cheaper than rotating a whole body to better look at stuff;
The legs... Azuaron suggested that a drider would have to have its legs below the body and not splayed. I propose a midterm solution which is close to the anatomy of a real spider, so the legs should be fixed below the cephalothorax, as in the picture below. A drider is supported by eight feet, and they don't have to be incredibly heavy for their size, so I think this setup works.
Source: this page at Deviant Art.
I estimate that, once the two first segments (head and "upper torso") reach the size of an adult human, the drider should be 6 feet tall and six feet long in a rest position, giving it an "L" shape somewhat. But it can put itself into much taller or shorter stances easily. It should weight 120 kilograms (approximately 265 pounds), with its weight divided almost evenly among its eight feet (the hindermost feet would support a smaller fraction of their weight), so on average each foot would be supporting 15 kg / 33 pounds.
(More mythical creature greatness. I found this picture in an image board. It had no source, but I'd bet my soul it comes from Drowtales. Go read it. Great webcomic with an awful lot of spiders in it.)
Such a drider would probably be warm-blooded, with all the adaptations required for that. It should eat at least as much as a human with the same weight.
Everything else is just cosmetic details. Here is how I would have it:
The head looks like a human's due to convergent evolution (evolution usually places most sensory organs in a head) and blind luck/coincidence. They probably have no external years and their noses will be kinda different from ours. The freakiest part may be them having six to eight eyes.
The upper torso looking human would be pure blind luck. They would probably be furry all over, even females, less than a spider but slightly more than a human. Females won't have breasts like mammals do.
Remember these beasts have external, not internal skeletons. Even if they develop a hide or skin in the upper torso that looks like ours, it will still have plates of chitin underneath. Their upper torso belly will be hard as a rock.
They could easily evolve hair out of the hair that spiders already have. Why they would evolve hair would be a mystery, just as it it a mystery to me why we have hair like we do.
Spiders have no actual brains... like insects they have ganglia. But the arachnid ganglia are all fused together and the bulk of it is located around the esophagus in the cephalotorax. A drider would have a very interesting neural anatomy, with a very long, diffuse "brain" surrounding the esophagus, going from the upper torso neck to the part where the original cephalotorax connects the abdomen.
Speaking of which, they don't have a backbone, so no medula as well. Also no ribs, if it wasn't clear enough when I mentioned chitin plates under the skin.
Last but not least, they are, as James pointed out, separated from humans by a long distance in the tree of life. Their physiology would be alien compared to ours. Any medicines and drugs will potentially have completely different effects on them compared to what they do to us. If you research enough into that you'll see that many invertebrates species have synapses and neurotransmitters different from those of vertebrates. In the very least, a doctor would have to learn medicine again practically from scratch to be able to help a drider.
A last, anecdotal commentary on that: spiders are so unlike humans, that coffee makes them less able to concentrate, but LSD makes them get into the flow somehow. I suppose that's because the arachnid brain already sees the world as a web of interconnected things or something like to that effect.