Paint.NET is free image and photo editing software for computers that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, andplugins.

It started development as an undergraduate college senior designproject 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 Microsoft Paint software that comes with Windows, it has grown into a powerful yet simple image and photo editor tool.

System Requirements

Minimum System

• Windows 7 (recommended), 
     or Windows XP SP3,
     or Windows Vista SP1 (SP2 and Platform Update recommended)
• 800MHz processor (dual-core recommended)
• 512MB 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 also depends on Microsoft’s .NET Framework 3.5 SP1, which is automatically installed if it isn’t already on the system.

Please note that Paint.NET uses your hard drive to store temporary files related to undo/redo history. Because of this, actual disk space requirements will depend on the actions you perform on an image, and on the size of the image.

Additional Resources; 

Blog  
Tutorials
Plugins
What’s new?
Frequently Asked Questions
System Requirements