Well, actually, I got my probleme solve using the authentication mechanisms in sharepoint side.
But still, I was starting to get results using something like that :
WebClient client =
sender, DownloadStringCompletedEventArgs e)
html.HtmlSource = e.Result;
The thing is, e.Result always raised an excpetion raising an error saying BrowserHttpStack can't handle custom credentials or something like that.
Maybe there is a workaround start in there, i don' tknow.