Call external web service

Hi,

I need call external web service, always go in error.....

my example:

<script src="http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.5.2.min.js"></script><script type="text/javascript">
        function testJson() {$.ajax({
                type: "POST",
                url: "http://xxx.com/ws2.asmx/HelloWorld",
                data: "{}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (msg) {
                    alert(msg.d)
                },
                error: function (msg) {
                    alert("ERROR")
                }
            });
        };</script><input type="button" name="test" value="test" onclick="testJson();" />

What is wrong in my source code?