The Kindle Fire may be an Android tablet, but it's not the tablet the way you think of an iPad or an Android-based tablet like the Motorola Xoom. It doesn't include the usual built-in Android apps, and its interface isn't primarily a way to run apps. Instead, it's mainly a way to purchase and use Amazon products and services, including books, movies, music, and more.
Windows 8 tablets will serve the exact opposite purpose. Their interfaces will be much like the current Windows Phone 7, a tile-based interface in which tiles deliver changing information, such as updates from social networking services, emails you've received, and so on. There will also be plenty of apps to run. To some extent, they'll be used for reading books and watching movies, but that's not their primary purpose.
Those looking primarily for a book reader, movie viewer, and music player will certainly want the Kindle fire. But they won't but one if they're looking for a tablet to read email and keep up with friends via social networking, and run apps. That's why people will buy a Windows 8 tablet. There's certainly some overlap between the two. But the heart of the market for each device is different, and so it's unlikely that the Kindle Fire will hurt Windows 8 tablets.