Windows Authentication with Ajax and Jquery

I'm creating an intranet application and I'm using jQuery to make web service calls:
$.ajax({ 
               type: "POST", 
               url: "WebService.asmx/" + fn,
               contentType: "application/json; charset=utf-8", 
               data: paramList,
               dataType: "json",
               success: successFn,
               error: errorFn
      ...