Problem with JSON in Internet Explorer 7

IE8/Chrome,FF work well but Internet Explorer 7 is giving me headaches.

I am trying to get numeric result for actual form

$(".checklist label").click(function () {
    checkResults();
});

function checkResults() {
    var str = $("form").serializeArray();
    $.ajax({
        type: "POST",
        url: "/data.asmx/GetTotal",
        cache: false,
        contentType: "application/json; charset=utf-8",
        data: JSON.stringify({ data: str }),
        dataType: "json",
        success: handleHtml,
        error: ajaxFailed
    });
}

function handleHtml(msg) {
    $("#result").text(msg.d);
}

function ajaxFailed(xmlRequest) {
}

What have I done wrong that IE7 wont work?

Thanks