ok scusa ci sono arrivato dopo:
codice:
        <script type="text/javascript">
            function changeSelect(str) {
                if (str == "") {
                    document.getElementById("categoria").innerHTML = "";
                    return;
                }

                var xmlhttp;
                var browser = navigator.appName;
                if(browser == "Microsoft Internet Explorer"){
                    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                } else {
                    xmlhttp = new XMLHttpRequest();
                }

                xmlhttp.onreadystatechange = function() {
                    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                        document.getElementById("div_cat").innerHTML = "<select name='categoria' id='categoria'><option value='*'>*</option>" + xmlhttp.responseText + "</select>";
                    }
                }
                xmlhttp.open("GET","home-cat.php?sid=" + str, true);
                xmlhttp.send();
            }
        </script>
....
            <tr>
                <td>Categoria:</td>
                <td>
                    <div id="div_cat">
                        <select name="categoria" id="categoria">
                            <option value='*'>*</option>
                        </select>
                    </div>
                </td>
            </tr>
grazie!!