(Not trying to turn this thread into an Editor war, but...) When I'm on Windows, I usually use gVim. However, for beginners, I'd recommend gedit. I heard some people also like Sublime Text these days, but it doesn't really give me any advantage over Vim.

The nice thing about Vim (and Emacs, for that matter) is that you can also use it directly on your device through the Terminal, the bad thing about both is that there's a non-trivial learning curve.

For editing on-device in a user-friendly way, try KhtSimpleText.