[POMAH-UST]

Skipp,
спасибо за помощь сделал так:

function opacity(e)
{
	var opacity = document.getElementById('opacity');
	var form = document.getElementById('form');
	if(e == true)
	{
		opacity.removeAttribute('class');
		form.removeAttribute('class');		
		var width = (document.documentElement.clientWidth - form.offsetWidth)/2;
		var height = (document.documentElement.clientHeight - form.offsetHeight)/2;		
		form.style.top = height + 'px';
		form.style.left = width + 'px';
		resize(true);
	}
	else
	{
		opacity.setAttribute('class', 'hidden');
		form.setAttribute('class', 'hidden');
		resize(false);
	}	
}
function resize(e)
{
	if(e == true) window.onresize = function () { opacity(1); }
	else window.onresize = function () { opacity(0); }
}