[devote]

function addEvent( elem, evt, callback ) {
	if ( elem.addEventListener ) {
		elem.addEventListener( evt, callback, false );
	} else if ( elem.attachEvent ) {
		elem.attachEvent( "on" + evt, callback );
	}
}

function clearEvent( e ) {
	e = e || window.event
	if (e.stopPropagation) e.stopPropagation();
	else e.cancelBubble = true;
	if (e.preventDefault) e.preventDefault();
	else e.returnValue = false;
}

window.onload = function() {

	var elem = document.getElementById('hoverthis');

	addEvent( elem, "mouseover", function() {
		var box = document.getElementById('box');
		box.className = 'active';
		box.style.display = 'block';
		return false;
	});
	addEvent( elem, "click", function( e ) {
		clearEvent( e );
		return false;
	});
	addEvent( document.getElementById('box'), "click", function( e ) {
		clearEvent( e );
		return false;
	});

	addEvent( document, "click", function() {
		var box = document.getElementById('box');
		box.className = '';
		box.style.display = 'none';
		return false;
	});
};

примерно так)