Try this
<script language="javascript" type="text/javascript">
function CalculateGPer()
{
var Form = document.frmReg;
var txt1= Form.txtBox1.value;
var txt2= Form.txtBox2.value;
var addition1= (Number(txt1)+Number(txt2));
if(!isNaN(Per) && Per!=Infinity)
{
Form.txtadd.value = addition1.toFixed(2);
}
return CheckZero()
}
</script>
Thanks
Ashish