http://blog.mdk-photo.com/post/The-most-versatile-httpRequest-function.aspx

Very often when you hand-code your AJAX, you'll need a good httpRequest function, which is easy to use/call and integrate.

here is my version:
function LoadURL(action,url,postbody,delegateReturn, isXML)
{
    var xmlhttp = false;

    try {    xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
    try {    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (E) {    xmlhttp = false; } }
    
    if (!xmlhttp && typeof(XMLHttpRequest) != 'undefined') {
    try {            xmlhttp = new XMLHttpRequest();
    } catch (e) {    xmlhttp = false;    }
    }
    if (!xmlhttp && window.createRequest) {
    try {            xmlhttp = window.createRequest();
    } catch (e) {    xmlhttp = false;    }
    }

    if ( xmlhttp )
    {
        xmlhttp.onreadystatechange = function(){
            if (xmlhttp.readyState==4){
                if (xmlhttp.status==200){
                    if(delegateReturn != null)
                    {
                        if(isXML){    delegateReturn( xmlhttp.responseXML );}
                        else{        delegateReturn( xmlhttp.responseText );}
                    }
                }else{
                    alert("Error: " + xmlhttp.status + "\n\nURL: "+URL);
                }
            }
        };
        try{
            xmlhttp.open(action,url, true);
            xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded;");
            xmlhttp.send(postbody);
        }catch(e){
            alert("xmlhttp error: \n" + e.message);
        }
    }else{
        alert("Your browser does not support XMLHTTP.");
    }
}

The parameters you'll need to pass:

action: "POST" or "GET"

url: The URL, which when using GET, holds the Querystring, and when POST'ing only the adress for the page you need to access

postbody: The Querystring of values, used in "POST - use encodeUriComponent() to encode the string, before sending it.

delegateReturn: The function pointer, for the function which will recieve the returned data, Null if you only want to send.

isXML: A boolean which tell the function, if the return value, is XML kind or Text kind. (you could ofcause re-write the function, to send the whole xmlhttp object to the return function)

- Its extremely easy to use:

function myReturnFunction(htmlDoc)
{
    doSomethingWithHTML( htmlDoc );
}

//Get data
LoadURL("GET", "page.php?query=value", null, myReturnFunction, false);

//Send data
var Data = "data=" + encodeUriComponent("This could be a complex string");
LoadURL("POST", "page.php", Data, null, false);