<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function activate(){
if(document.getElementById('agree').checked==true){document.getElementById('sendbutton').disabled=false;}
else{document.getElementById('sendbutton').disabled=true;}}
</script>
</head>
<body>
<input type="submit" id="sendbutton" disabled />
<input onclick="activate();" type="checkbox" id="agree" /><label for="agree">I agree</label>
</body>
</html>