FlowDocument document = new FlowDocument();
Paragraph paragraph = new Paragraph();
Hyperlink link = new Hyperlink(new Run("Ссылка"));
link.NavigateUri = new Uri("http://sql.ru/forum/actualpost.aspx?bid=34&tid=697955&mid=7698869&p=1");
link.RequestNavigate += new RequestNavigateEventHandler(delegate(object obj, RequestNavigateEventArgs nav)
{
    Process.Start("IExplore.exe", nav.Uri.OriginalString);
});
paragraph.Inlines.Add(link);
document.Blocks.Add(paragraph);
control.Document = document;