For what it's worth, I do a lot of my first draft work with pencil and paper. Old school enough for you?
The truth is that writing requires almost no technology. However, technology *can* make it easier. Today I took my new laptop to a coffee house to work for a few hours away from the distractions at home. This was easy to do because it's so light and easy to carry. I then worked for several hours straight in the corner I preferred, without having to drag out a heavy power brick and look for an outlet. By the time I left, my battery was at 94%. Try doing that with a craptop. Granted, this was without using wireless, but I'm still very pleased. The clarity of the screen also helps my concentration and productivity. And while Scrivener does exist for Windows these days, it's still behind the Mac version a bit, and Scrivener is the best writing software I've run across.
Yes, it's probably overkill. On the other hand, I don't have a problem with embracing technology that improves my life. You *can* run businesses with no bookkeeping software or smart phones with credit card readers or any other new tools. People did it for ages. But if there's technology that makes it easier, why not use it?
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I support this response to the posts that recommend any old craptop will do for writing.
Sure, I could use a typewriter or even a quill, I could also have a serious operation without anaesthetic, and travel 500km to the nearest major city by horse and cart.
Like most professional writers I use my computer for other things too, and what I crave most is a good screen, followed by portability and good battery life.
An MBPr is not overkill for me. It's silly to judge suitability merely on CPU power.