call web service

Dear forum.

i try to access to a resource in the server,via Javascipt whit XMLHttpRequest() object.

Just for begin, i access to a text file named -TextFile1.txt-, and is working very easy and at first time. This is the code:

if (window.XMLHttpRequest)
{
xmlhttp=new XMLHttpRequest() ;
xmlhttp.onreadystatechange=EV_RISP; //Event handler
xmlhttp.open("get", "TextFile1.txt", true); //Source to access
xmlhttp.send() ;
}

and the event:
function EV_RISP() {

if (xmlhttp.readyState == 4) {
alert(xmlhttp.responseText);
}

else {
alert("NOK");
}
}

Now i try to access to a web service in ASP.NET ,the simple HelloWorld
but is not working
The web service is alwai the same:

public class WebService1 : System.Web.Services.WebService
{

[WebMethod]
public string HelloWorld()
{
return "Hello-World";
}
}

and my code in javascript is modified just in the line of destination source

xmlhttp.open("get", "webservice1/HelloWorld", true);

and i get the NOK of the alert code.

Many thanks
Roberto