Ještě že mě láásko mááš....
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=windows-1250">
<title>test</title>
<script type="text/javascript">
function Zamena(value) {
if(value == "") return;
value = value.replace(',', '.');
alert(value);
if(parseFloat(value) != (value*1)) return;
return true;
}
</script>
</head>
<body>
<input type="text" id="zadat" onclick="Zamena(this.value)" value="" />
<input type="text" />
</body>
</html>