[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();
    document.LoadXml(
@"<app-settings><BackgroundColor>Red</BackgroundColor></app-settings>");
    AuthenticatedConnection.SetApplicationSettings(document);

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