Here's a great looking and feature rich Markdown editor for Windows called WriteMonkey. It's a portable application, which means no installation is necessary.
What's more, it is beautifully simple. On first launch you get a black window with a blinking cursor. There's no menu or other chrome by default. Hitting F1 brings up a simple keyboard shortcut screen that also offers Markdown syntax reminders.
F10 opens the preferences dialog. The application is deceptively simple. The preferences reveal a huge amount of features such as text replacement shortcuts web look-ups of selected text, scratch repository and dynamic Markdown shortcuts. This application is a phenomenal implementation of a simple Markdown editor
The best description of this application is that it is WriteRoom for windows but with the Markdown support of NVAlt and the feature set of TextWrangler. If you know what all that means and you work on Windows, then go download this FREE application. Please donate to the developer if you find it useful.