function addLoadEvent(func) {   
    var oldonload = window.onload;
    if (typeof window.onload != 'function'){
        window.onload = func;
    } else {
        window.onload = function(){
        oldonload();
        func();
        }
    }
}

function chargement()
{
	var elem = document.getElementById('Menu');
	var children = elem.childNodes;
	var nbChildren = children.length;
	var i = 0;
	for(i = 0; i < nbChildren; i++)
	{
		if(children[i].className == "MenuItem")
		{
			children[i].onmouseover = function()
			{
				this.className = 'MenuItemActif';
			}
			children[i].onmouseout = function()
			{
				this.className = 'MenuItem';
			}
		}
	}
}

addLoadEvent(chargement);