If you are planning to develop InfoPath forms and execute develop your login using C# or vb.net you must download the InfoPath 2003 Toolkit for Visual Studio.

The InfoPath 2003 Toolkit for Visual Studio .NET provides integration features and documentation that enable you to use Visual Studio .NET to create, debug, and build InfoPath projects that use Visual C# or Visual Basic .NET managed code. The integration features provided by the toolkit allow you to use a combination of InfoPath for form design and Visual Studio .NET for writing and debugging form code. Using the toolkit, you can create new InfoPath form templates or add managed code to existing templates.