[Matre]

<input type='button' value='отправить' onclick='Request();' />
<input type='checkbox' id='chck' />
<input type='text' id='txt' />
<div id='result'></div>
<script type='text/javascript'>
	function Request() {
		var url = document.getElementById("chck").checked ? "url если checkbox отмечен" : "или не отмечен";
		var request = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
		request.open("POST", url, true);
		request.onreadystatechange = function () {
			if (request.readyState + request.status === 204)
				document.getElementById("result").innerHTML = request.responseText;
		};
		request.send("field=" + encodeURIComponent(document.getElementById("txt").value));
	}
</script>