L'élément document et le reste...

var alerting = Class.create({
	initialize: function(){
	},
	alerter: function(condition){
		if(condition == true){
			alert('alert');
		}
	}
});

Event.observe(window,'load',function (){
	Event.observe(document,'click',function(){
		var a = new alerting();
		a.alerter(true);
	});
	
	Event.observe($('menu_nv_1'),'click',function(){
		var a = new alerting();
		a.alerter(false);
	});
});									  

var alerting = Class.create({
	initialize: function(obj){
		this.obj = obj;
	},
	alerter: function(condition){
		if(condition == true){
			this.obj.onclick = new Function ("alert('exemple parmis tant dautre')");
		}
	}
});

Event.observe(window,'load',function (){
	var a = new alerting(document);
	a.alerter(true);									  
	var b = new alerting($('menu_nv_1'));
	b.alerter(false);									  
	var c = new alerting($('menu_nv_2'));
	c.alerter(false);									  
});	

var alerting = Class.create({
	initialize: function (obj, message) {
		this.obj = obj;
                // obj_.param = param_; // Pour une raison d'habitude j'ai préféré faire
		this.message = message; // ceci (+ voir "cela")
	},
	alerter: function (condition) {
		if (condition == true) {
			Event.observe(this.obj, 'click', function (e) {
				alert('onclick :\n' + this.message);
				e.stopPropagation();
			}.bind(this)); // cela
		}else{
			Event.observe(this.obj, 'click', function (e) { //Pour les remarques 2 et 3
				e.stopPropagation();                    //Pour les remarques 2 et 3
			});                                             //Pour les remarques 2 et 3
		}
	}
});

Event.observe(window, 'load', function () {
	var a = new alerting(document, 'Click sur document');
	a.alerter(false);
	var b = new alerting($('menu_nv_1'), 'Hello! de menu_nv_1');
	b.alerter(true);
	var c = new alerting($('menu_nv_2'), 'Hello! de menu_nv_2');
	c.alerter(true);
});

Event.observe(window, 'load', function () {
	var a = new alerting(document, 'Click sur document');
	a.alerter(false); //ici
	var b = new alerting($('menu_nv_1'), 'Hello! de menu_nv_1');
	b.alerter(true);  //ici
	var c = new alerting($('menu_nv_2'), 'Hello! de menu_nv_2');
	c.alerter(true);  //ici
});

Event.observe(window, 'load', function () {
	var a = new alerting(document, 'Click sur document');
	a.alerter(true);   //la
	var b = new alerting($('menu_nv_1'), 'Hello! de menu_nv_1');
	b.alerter(false);  //la
	var c = new alerting($('menu_nv_2'), 'Hello! de menu_nv_2');
	c.alerter(false);  //la
});

e.stop();

e.stopPropagation();