dataType应该是您收到的内容的类型，但contentType应该是您要发送的内容的哑剧类型，以下应该可以：

$(document).ready(function() {
        $.ajax({ type: "POST",
                        url: "/getprojects.ashx",
                        data: "<formData client=\"\" year=\"\" categories=\"\" tags=\"\" freeText=\"\" count=\"34\" page=\"1\"></formData>",
                        contentType: "text/xml",
                        dataType: "xml",
                        cache: false,
                        error: function() { alert("No data found."); },
                        success: function(xml) {
                            alert("it works");
                            alert($(xml).find("project")[0].attr("id"));
                        }
        });
    });