Java banner rotator and Nofollow

Hello, I'm new here
I have a question: how can I add the nofollow tag to the links I have in this banner rotator?

<script type="text/javascript">
<!-- Begin
rnd.today=new Date();
rnd.seed=rnd.today.getTime();

function rnd() {
        rnd.seed = (rnd.seed*9301+49297) % 233280;
        return rnd.seed/(233280.0);
};

function rand(number) {
	var result = Math.ceil(rnd()*number);
	if (!result)result++;
        return result
};
var ad_cnt2 = 2;
var ad2 = rand(ad_cnt2);
var link2;
var adBanner2;
var width2
var height2
if (ad2==1) {
link2="http://TheSpiderWebSystem.com/greatnews";
adBanner2="http://thespiderwebsystem.net/spiderweb/s/728x90_wide.gif";
width2="728";
height2="90";
alt2="";
}
if (ad2==2) {
link2="/paidads.html";
adBanner2="/headerbanner.gif";
width2="728";
height2="90";
alt2="";
}
document.write('<center><a href="' + link2 + '" target="_blank">');
document.write('<img src="' + adBanner2 + '" width=' + width2 + ' height=' + height2 + ' border=0 alt="' + alt2 + '"></a>');
document.write('</center>');
// End -->
</SCRIPT>

Code (markup):

Thank you!!

 

At the bottom

[b]document.write('<center><a href="' + link2 + '" target="_blank" rel="nofollow">');[/b]
document.write('<img src="' + adBanner2 + '" width=' + width2 + ' height=' + height2 + ' border=0 alt="' + alt2 + '"></a>');
document.write('</center>');
// End -->
</SCRIPT>

Code (markup):