function changeVideo(id, category) {

	var li = document.getElementById("video_" + id);

	var xhr_object;
	if(window.XMLHttpRequest) // Firefox
		xhr_object = new XMLHttpRequest();
	else if(window.ActiveXObject) // Internet Explorer
		xhr_object = new ActiveXObject("Microsoft.XMLHTTP");
	else
		return false;

	xhr_object.open("GET", "thumb.php?category=" + category, true);
	xhr_object.onreadystatechange = function()
	{
		if(xhr_object.readyState == 4)
			li.innerHTML = xhr_object.responseText;
	}
	xhr_object.send(null);

	id ++;
	id %= 4;
	setTimeout("changeVideo(" + id + ", " + category + ")", 15000);
}