In addition to the public XML Web Service (through which all HealthVault traffic flows), Microsoft also has developed and made available a .NET SDK that can be used to develop HealthVault-connected applications.  The SDK provides a friendly object model and higher-level abstractions that take care of a considerable amount of work for you, reducing development time and costs.

You can download the .NET SDK here: http://www.microsoft.com/downloads/details.aspx?familyid=95E14343-FB98-4549-BD29-225A59423CC9&displaylang=en.

The Online .NET Reference Library is also available.

And, of course, many other kinds of information are available via the MSDN HealthVault Developer Center.

For specific .NET SDK topics, check out the Table of Contents in the right-hand navigation.