var isNS4 = (document.layers) ? true : false;
var isDOM = document.getElementById //DOM1 browser (MSIE 5+, Netscape 6, Opera 5+)
var isIE5 = (document.all && isDOM) ? true : false; // IE5
var isNS6 = (!document.all && isDOM) ? true : false; // NS6
var isOpera = window.opera //Opera
var isOpera5 = isOpera && isDOM //Opera 5+
var isOpera6=isOpera && window.print //Opera 6+
var isOpera7=isOpera && document.readyState //Opera 7+
var isMozilla=isDOM && navigator.appName=="Netscape"

function getWindowWidth() {
	var windowWidth = 0;
	if (typeof(window.innerWidth) == 'number') {
		windowWidth = window.innerWidth;
	}
	else {
		if (document.documentElement && document.documentElement.clientWidth) {
			windowWidth = document.documentElement.clientWidth;
		}
		else {
			if (document.body && document.body.clientWidth) {
				windowWidth = document.body.clientWidth;
			}
		}
	}
	return windowWidth;
}

function getWindowHeight() {
	var windowHeight = 0;
	if (typeof(window.innerHeight) == 'number') {
		windowHeight = window.innerHeight;
	}
	else {
		if (document.documentElement && document.documentElement.clientHeight) {
			windowHeight = document.documentElement.clientHeight;
		}
		else {
			if (document.body && document.body.clientHeight) {
				windowHeight = document.body.clientHeight;
			}
		}
	}
	return windowHeight;
}

function setContent() {
	if (document.getElementById) {
		var windowWidth = getWindowWidth();
		if (windowWidth > 0) {
			var contentElement = document.getElementById('Menu');
			var contentWidth = contentElement.offsetWidth;
			if (windowWidth - contentWidth > 0) {
				contentElement.style.position = 'relative';
				contentElement.style.visibility = 'visible';
				//contentElement.style.left = ((windowWidth / 2) - (contentWidth / 2)) + 'px';
			}
			else {
				contentElement.style.position = 'relative';
				contentElement.style.visibility = 'visible';
				contentElement.style.left = ((windowWidth / 2) - (contentWidth / 2)) + 'px'
			}
		}
		
		var windowHeight = getWindowHeight();
		if (windowHeight > 0) {
			var contentElement = document.getElementById('Menu');
			var contentHeight = contentElement.offsetHeight;
			if (windowHeight - contentHeight > 0) {
				contentElement.style.position = 'relative';
				contentElement.style.visibility = 'visible';
				contentElement.style.top = ((windowHeight / 4) - (contentHeight / 2) + 40) + 'px';
			}
			else {
				contentElement.style.position = 'relative';
				contentElement.style.visibility = 'visible';
				contentElement.style.top = ((windowHeight / 4) - (contentHeight / 2) + 40) + 'px';
			}
		}
	}
}
<!--window.onload = function() {-->
<!--	setContent();-->
<!--}-->
<!--window.onresize = function() {-->
<!--	setContent();-->
<!--}-->