.net question - where are the DefaultCredentials stored/accessed from for a WinForms v3.5 app?

Where are the DefaultCredentials stored/accessed from for a WinForms v3.5 app?

That is if I am using the settings for defaultProxy for my Winforms v3.5 application, and set a proxy server address here, exactly where does/can the username/password come from? Or in other words where does the framework source the "default credentials" for a winforms application running on the client PC?

<defaultProxy
  enabled="true|false"
  useDefaultCredentials="true|false"
  <bypasslist> … </bypasslist>
  <proxy> … </proxy>
  <module> … </module>
/>

Background - apparently ClickOnce can use this for a client side application, however I'm trying to work out where click once would get this defaultCredential from, for a user who is running the clickonce install for my winforms application.

PS Hopefully the answer is not IE, and if so the next question is how to set it for users that use Firefox as their browser开发者_如何学JAVA of choice.