<input type="text" name="aa" id="aa" onchange="cc.value=this.value+bb.value">
<input type="text" name="bb" id="bb" onchange="cc.value=this.value+aa.value">>
<input type="text" name="cc" id="cc">
<input type="text" name="bb" id="bb" onchange="cc.value=this.value+aa.value">>
<input type="text" name="cc" id="cc">
<input onpropertychange="cc.value=parseFloat(aa.value)+parseFloat(bb.value)" name="bb" value="0">
<input name="cc" value="">
<input type="text" name="aa">
<input type="text" name="bb">
<input type="text" name="cc">
<script>
function add()
{
document.all.cc.value=parseInt(document.all.aa.value)+parseInt(document.all.bb.value);
setTimeout('add()',100)
}</script>