[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.