[Sweet]

<form name="form1">
  <input type="text" onclick="this.style.backgroundColor = 'white';" name="visota" /><br/>
  <input type="text" onclick="this.style.backgroundColor = 'white';" name="shirina" /><br/>
  <input type="button" onclick="raschet(this.form)" value="Перемножить" /><br/>
  <input type="text" name="res_metall" />
</form>
<script>
function raschet(form){
  var visota = form[0], shirina = form[1],
  error;

  if(isNaN(visota.value)) {
    visota.style.backgroundColor = 'red';
    error = true;
  }
  if(isNaN(shirina.value)) {
    shirina.style.backgroundColor = 'red';
    error = true;
  }
  if(error == true) return NaN;
  s = visota.value * shirina.value;
  form[3].value=s;
  return s;
}
</script>