onload = function() {
    var divl = document.documentElement.clientWidth;
    if (parseInt(divl) > 1024) {
        document.getElementById('banner').style.display = "block";
    } else {
        document.getElementById('banner').style.display = "none";
    }
}

window.onresize = function() {
    var divl = document.documentElement.clientWidth;
    //alert(divl);
    var bannerwidth = document.getElementById('banner').style.width;
    /*
    dit is de inner-breedte van de pagina + de breedte van de banner + de margin + 5 extra pixels
    om te voorkomen dat de banner onder de pagina schuift
    */
    if (parseInt(divl) > (1024 + parseInt(bannerwidth)) + 105) {
        document.getElementById('banner').style.display = "block";
    } else {
        document.getElementById('banner').style.display = "none";
    }
    //document.getElementById('banner').innerHTML = divl;
}

function getScrollXY() {
    var scrOfX = 0, scrOfY = 0;
    if (typeof (window.pageYOffset) == 'number') {
        //Netscape compliant
        scrOfY = window.pageYOffset;
        scrOfX = window.pageXOffset;
    } else if (document.body && (document.body.scrollLeft || document.body.scrollTop)) {
        //DOM compliant
        scrOfY = document.body.scrollTop;
        scrOfX = document.body.scrollLeft;
    } else if (document.documentElement && (document.documentElement.scrollLeft || document.documentElement.scrollTop)) {
        //IE6 standards compliant mode
        scrOfY = document.documentElement.scrollTop;
        scrOfX = document.documentElement.scrollLeft;
    }
    return [scrOfY];
}

window.onscroll = scrollEvent;
function scrollEvent() {
    var docH = document.getElementById("main_content").offsetHeight - 600;
    document.getElementById('banner').style.position = "absolute";
    if (getScrollXY() < docH) {
        document.getElementById('banner').style.marginTop = getScrollXY() + "px";
    }
}