codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta http-equiv="Content-Language" content="it" />
<title>Esempio per il Forum di HTML.it</title>
<script type="text/javascript">
function checkInput(oMyField, oKeyEvent) {
	var nChar = oKeyEvent.charCode, sChar = String.fromCharCode(nChar), rSeparator = /,/;
	return nChar === 0 || /\d/.test(sChar) || (rSeparator.test(sChar) && !rSeparator.test(oMyField.value));
}

function checkValue(oMyField) {
	oMyField.value = oMyField.value.replace(/,$/, "").replace(/^,/, "0,");
}
</script>
</head>

<body>

<form name="form1">


<input type="text" name="importo1" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="importo2" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="importo3" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="importo4" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="importo5" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="importo6" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" />

<input type="text" name="nomeACaso" onkeypress="return(checkInput(this, event));" onblur="checkValue(this);" onpaste="return(false);" /></p>
</form>

</body>
</html>