Edit in JSFiddle

$("input[type=checkbox]").click(function(){
    var $count = $("input[type=checkbox]:checked").length;
    var $not = $('input[type=checkbox]').not(':checked')

    if($count >= 1) {
        $not.attr("disabled",true);
    }else{
        $not.attr("disabled",false);
    }
});

<input type="checkbox" value="1"><br />
<input type="checkbox" value="1"><br />
<input type="checkbox" value="1"><br />
<input type="checkbox" value="1"><br />