function showModalWindow(id) {
	var maskHeight = $(document).height()+'px';
	var maskWidth = $(window).width()+'px';
	$('#mask').css({'width':maskWidth,'height':maskHeight});
//transition effect		
	$('#mask').fadeIn(1000).fadeTo("slow",0.8);	
//Get the window height and width
	var winH = $(window).height();
	var winW = $(window).width();
	//Set the popup window to center
	$(id).css('top',  $(window).scrollTop()+winH/2-$(id).height()/2);
	$(id).css('left', $(window).scrollLeft()+winW/2-$(id).width()/2);
	$(id).hide();
	$(id).fadeIn(2000); 
}