if (document.layers) {
    document.captureEvents(Event.KEYDOWN);
}

document.onkeydown = function (evt) {
    var keyCode = evt ? (evt.which ? evt.which : evt.keyCode) : event.keyCode;

    //13 = the code for pressing ENTER
    if (keyCode == 13) {
        if (window.formEnterSubmit) {
            formEnterSubmit();
        }
    }
}