Have you checked out Puppy, Tiny Core, or antiX? They're all designed for low-powered machines. I haven't tried Tiny Core yet, but Puppy and antiX are very usable and have active communities around them. If what you want to do is write, you don't really need a word processor. A basic text editor like Leafpad should be fine, or if you want something with a little more functionality, try FocusWriter.