[Update: Removed PB3 reference - this has been around for a while]

HealthVault provides a way for applications to store information associated with the current user. This is done using the ApplicationSettings class, which can be get and set through the AuthenticatedConnection type.

Here's a few lines of code to get you started:

    ApplicationSettings settings = AuthenticatedConnection.GetAllApplicationSettings();

    XmlDocument document = new XmlDocument();

The outer tag must be "app-settings", but the format inside of that is up to the application.