nurik2120,
Вариант ...
<!DOCTYPE HTML>
<html>
<head>
<title></title>
<meta charset="utf-8" />
</head>
<body>
<select name="select_left" size="1" id="select_left_id">
<option selected="selected" value="avto">Автомобили</option>
<option value="moto">Мотоциклы</option>
<option value="shlemi">Одежда шлемы</option>
<option value="furgon_do">Фургоны, грузовые автомобили до 7,5т</option>
<option value="gruz_sv">Грузовые автомобили свыше 7,5т</option>
</select>
<div id="avto" style="display: block;">avto</div>
<div id="moto" style="display: none;">moto</div>
<div id="shlemi" style="display: none;">shlemi</div>
<div id="furgon_do" style="display: none;">furgon_do</div>
<div id="gruz_sv" style="display: none;">gruz_sv</div>
<script>
(function () {
var c = document.getElementById("select_left_id"),
a = document.getElementById("avto");
c.onchange = function () {
var b = document.getElementById(this.value);
b.style.display = "block";
a.style.display = "none";
a = b
}
})();
</script>
</body>
</html>