var doHotelprontoX = (screen.width/2)-405;
var doHotelprontoY = (screen.height/2)-300;
var pos = "left="+doHotelprontoX+",top="+doHotelprontoY;

function doHotelpronto(){
doHotelprontoWindow = window.open("http://www.hotelpronto.com/Customer/Availability/HotelSearch.aspx?AffiliateId=11611&lang=nl-NL","Bart","location=yes,directories=yes,toolbar=yes,menubar=yes,status=yes,scrollbars=yes,resizable=yes,width=810,height=600,"+pos);
}