a sto punto mi sa che sto facendo qualche errore banalissimo... cmq:

file form.php
Codice PHP:
<html>
    <head>
        ...cut...
    </head>
    <body>
        <div id="content">
            <form action="anteprima.php" method="post">
                                ...cut...
                [url="#"]prova[/url]
                ...cut...
            </form>
        </div>
    </body>
</html> 
file script.js
Codice PHP:


var url     = "./visualizza_immagini.php";

function sendPath(path){
    if (httpSetImages.readyState == 4 || httpSetImages.readyState == 0){    
        param = 'path=' + path; 
        httpSetImages.open('POST', url, true);
    httpSetImages.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
        //httpSetImages.setRequestHeader('Connection','close');
        httpSetImages.onreadystatechange = handleHttpSetImages;
        httpSetImages.send(param);
    }
}

//initiates the XMLHttpRequest object
//as found here: [url]http://www.webpasties.com/xmlHttpRequest[/url]
function getHTTPObject(){
    var xmlhttp;
    /*@cc_on
    @if (@_jscript_version >= 5)
    try {
    xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
    try {
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    } catch (E) {
    xmlhttp = false;
    }
    }
    @else
    xmlhttp = false;
    @end @*/
    if (!xmlhttp && typeof XMLHttpRequest != 'undefined') {
        try{
            xmlhttp = new XMLHttpRequest();
        } catch (e){
            xmlhttp = false;
        }
    }
    return xmlhttp;
}

var httpSetImages = getHTTPObject(); 
file visualizza_immagini.php:
Codice PHP:
<?php
print_r($_POST);
print_r($_GET);
print_r($_REQUEST);