Paint.NET is image and photo manipulation software designed to be used on computers that run Windows. It supports layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. It started development as an undergraduate college senior design project mentored by Microsoft, and is currently being maintained by some of the alumni that originally worked on it. Originally intended as a free replacement for the MS Paint software that comes with Windows, it has grown into a powerful yet simple tool for photo and image editing. The programming language used to create Paint.NET is C#, with a small amount of C++ for installation and shell-integration related functionality.

I would personally describe as, 'it is a simplified and basic version of Adobe PhotoShop'.
Also, it has additional plugins you can download and use from: http://paintdotnet.forumer.com/index.php

 System Requirements
   * Windows 7 (recommended), Windows XP SP2, or Windows Vista
   * .NET Framework 3.5 SP1
   * 512 MB of RAM
   * 1024 x 768 screen resolution
   * 200+ MB hard drive space
   * Optional: 64-bit mode requires a 64-bit CPU and a 64-bit edition of Windows

Paint.NET 3.5 Download link: http://www.getpaint.net/download.html
.NET Framework 3.5SP1 download link: http://go.microsoft.com/fwlink/?LinkId=124150

I know it was released a while ago but I just realized today..  
Hope it was useful info for those who wants a free photo editing software other than GIMP, etc.