Self authentication in ASP.NET

Hi, I'm making a web app that accesses a server to get images. I already have the username and password and don't wish for the users to be asked for it when they open the web app. I need a self-authenticating script that would pass these credentials to the server automatically. I was able to achieve this in javascript but need an implementation in ASP.NET. Following is the javascript implementation:
    var xmlHttp=new XMLHttpRequest();
    xmlHttp.mozBackgroundRequest = true;
    xmlHttp.open("GET",URL,true,username,password);
    xmlHttp.send(null);
Any help would be good. Thanks.

...