Code:
<div id="banners" onclick="trackclick();">

<script language="JavaScript">

if ( clickCount > 2 ){
    document.write('
        <a href="http://www.pjtra.com/t/RD1FRklFPURASUk9RURISQ">
        <img src="http://www.pjtra.com/b/RD1FRklFPURASUk9RURISQ?sid=AFFBEST">
        </a>
        ');
}else{
    document.write('
        <script type="text/javascript"><!--
    google_ad_client = "pub-8299655316220106";
    /* C4G-Forum-728x90, created 5/9/08 */
    google_ad_slot = "3304568502";
    google_ad_width = 728;
    google_ad_height = 90;
    //-->
    </script>
    <script type="text/javascript"
    src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
    </script>
        ');
}

</script>
</div>



<script language="JavaScript">

var clickCount = readCookie();
if ( clickCount == null ){
    clickCount = 0;
}

function trackclick() {
    clickCount = parseInt(clickCount)+1;
    setCookie( clickCount, 2 );
    return true;
}

function setCookie( cookieValue, nDays) {
    var today = new Date();
    var expire = new Date();
    if (nDays==null || nDays==0) nDays=1;
    expire.setTime(today.getTime() + 3600000*24*nDays);
    document.cookie = "clicktrack="+escape(cookieValue)
                      + ";expires="+expire.toGMTString();
}

function readCookie() {
    var nameEQ = "clicktrack=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(nameEQ) == 0){
            return c.substring(nameEQ.length,c.length);
        }
    }
    return null;
}

</script>