function uncheckCheckboxGroup(groupName) {
    var el = document.forms[0].elements;
    for(i=0; i<el.length; i++) {
        if(el[i].type == "checkbox" ) {
            if(el[i].className == groupName) {
                el[i].checked = false;
            }
        }
    }
}

function uncheckAllCheckboxes() {
    var el = document.forms[0].elements;
    for(i=0; i<el.length; i++) {
        if(el[i].type == "checkbox" ) {
          el[i].checked = false;
        }
    }
}