Something

<html>
<head>
<title>test</title>
<script type='text/javascript'>
function check()
 {
   var text = document.getElementById('LName') ;
   var sub = document.getElementById('submit') ;
   if (text.value != "" && text2.value != "")
  {
    sub.disabled = false
  }
}
</script>
</head>
<body>
<form>
<input type="text" onchange="return check()" id="LName" >
<input type="submit" value="submit" id="submit" />
</form>
</body>
</html>