[Sweet]

Или так:

<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 check(el){
  if(isNaN(el.value)) {
    el.style.backgroundColor = 'red';
    return false;
  } else return true;
}
function raschet(form){
  var visota = form[0], shirina = form[1],
  checkResult = [check(visota), check(shirina)];
  for(var i = 0; i < checkResult.length; i++) if(!checkResult[i]) return NaN;
  form[3].value=visota.value * shirina.value;
}
</script>