Swf2XNA is an open source program under BSD license, developed in C#, with it , you can make Windows/XBox/Win7 games by leveraging Flash for assets and layout, as well as using Box2D visually. It makes 2D games in XNA quite simple to make!
Download source code here.
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About XNA
Microsoft® XNA™ is composed of industry-leading software, services, resources, and communities focused on enabling game developers to be successful on Microsoft gaming platforms.