Un ejemplo tonto, para IE
Código:
<html>
<head>
<meta http-equiv="Content-Language" content="es">
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<title>Pagina nueva 1</title>
<script>
function calcula(){
var total=0
total=parseInt(window.T2.value)
total+=parseInt(window.T3.value)
total+=parseInt(window.T4.value)
window.T5.value=total
}
</script>
</head>
<body>
<p><input type="text" name="T2" size="20" value="12" onkeyup="calcula()"></p>
<p><input type="text" name="T3" size="20" value="23" onkeyup="calcula()"></p>
<p><input type="text" name="T4" size="20" value="-4" onkeyup="calcula()"></p>
<p>_____________________+</p>
<p><input type="text" name="T5" size="20"></p>
</body>
</html>