Prova questo (IE)
Inserisci un numero e poi clicca sul pulsante.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
function completeNumField(fieldName,digitInt,digitDec) {
 fieldName.value = fieldName.value.replace(',','.');
 //----------- Sortie s'il ne s'agit pas d'une donnée numérique
 if (isNaN(fieldName.value)) {return;}
 //----------- Si la longueur est atteinte on sort
 if (fieldName.value.length >= (digitInt+digitDec)) {return;}
 //----------- Recherche de . ou ,
 var re = /(\.)/i;
 found  = fieldName.value.search(re);
 if (found == -1) {
  re    = /(,)/i;
  found = fieldName.value.search(re);
 } // if (found == -1)
 if (found != -1) {
  //-------------- Nombre avec virgule
  var integerPart = fieldName.value.substr(0,found);
	var decimalPart = fieldName.value.substr(found+1,fieldName.value.length - found - 1);
	//----- Partie décimale
	if (decimalPart.length > digitDec) {
	 decimalPart  = decimalPart.substr(0,digitDec -1);
	} else {
	 for (i=decimalPart.length;i<digitDec;i++) {
	  decimalPart = decimalPart + '0';
	 } // if (decimalPart.length > digitDec)
	} // if (decimalPart.length > digitDec)
	//----- Partie entière
	for (i=integerPart.length;i<digitInt;i++) {
	 integerPart = '0' + integerPart;
	} // for (i=integerPart.length;i<digitInt;i++)
	fieldName.value = integerPart + decimalPart;
	return;
 } else {
	//----- Partie décimale
	for (i=0;i<digitDec;i++) {
	 fieldName.value = fieldName.value + '0';
	} // if (decimalPart.length > digitDec)
	//----- Partie entière
	for (i=fieldName.value.length;i<(digitInt+digitDec);i++) {
	 fieldName.value = '0' + fieldName.value;
	} // for (i=integerPart.length;i<digitInt;i++)
	return;
 } // if (found != -1)
 alert(integerPart + " " + decimalPart);
 return;
// fieldName.value = "";
}

//-->
</script>

<input type="text" id="text1">
<input type="button" value="clicca" onclick="completeNumField(document.getElementById('text1'),8,0)">
</body>
</html>