checkbox_clicked = function(sender, chk)
{
	var f = document.reg_form;
	var n = sender.name;
	if (typeof f.elements[n] != 'undefined') {
  	if (typeof f.elements[n][0] != 'undefined'){
			//there are many checkboxes
			for(i=0; i<f.elements[n].length; i++) {
				f.elements[n][i].checked = false;
			}
		}
  	else {
			//there's only one checkbox
			f.elements[n].checked = false;
  	}
	}
	if(!chk)
		sender.checked = false;
	else
		sender.checked = true;
}

/* old version
checkbox_clicked = function(sender, chk)
{
	var f = document.reg_form;
	var n = sender.name;
	if (typeof f.elements[n] != 'undefined') {
  	if (typeof f.elements[n][0] != 'undefined'){
			//there are many checkboxes
			for(i=0; i<f.elements[n].length; i++) {
				f.elements[n][i].checked = false;
			}
		}
  	else {
			//there's only one checkbox
			f.elements[n].checked = false;
  	}
	}
	if(!chk)
		sender.checked = false;
	else
		sender.checked = true;
}
*/