PageMethods - Authentication Required box

Hi,
I've just written my first pagemethod for a page:
<script type="text/javascript">
//<![CDATA[
function GetTime() {
alert(PageMethods.GetTime());
}
//]]>
</script>
[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod(UseHttpGet = true)]
public static string GetTime()
{
    return DateTime.Now.ToUniversalTime().ToString();
}
Whenever my javascript calls the PageMethod (it's just an example above) I get a popup box saying Authentication Required, Enter user name and password for http://localhost!
Please can someone help cos I'm going mad ... Thanks
...