Code:
document.write ("<p id=ad"+ADid+">LOADING</p>");
adGrab(ADtags, ADtitle);

function getHTTPObject(){ if(typeof XMLHttpRequest!='undefined') { return new XMLHttpRequest();} 

try{ return new ActiveXObject("Msxml2.XMLHTTP");} catch (e){try{ return new 

ActiveXObject("Microsoft.XMLHTTP");} catch (e) {}} return false;}

function adGrab(ADtags, ADtitle) {
 myurl = "http://ads.ismywebsite.com/ad.php?title="+ADtitle+"&tags="+ADtags+"&t="+Date.parse(new 

Date());
 http = getHTTPObject();
 alert (1);
 http.open("GET", myurl, true);
 alert (2);
 http.onreadystatechange = adShow;
 alert (3);
 http.send(null);
 alert (4);
}

function adShow() {
 alert (5);
 if (http.readyState == 4) {
  var text = http.responseText;
  alert (6);
  alert("document.getElementById('ad"+ADid+"').innerHTML = '"+text+"'");
  alert (7);
  var name = 'ad'+ADid;
  alert (name+"-"+text);
  if (document.getElementById) {
   document.getElementById(name).innerHTML = text;
   alert (8);
  }
  else if (document.all) { document.all[name].innerHTML = http.responseText; alert (9);}
  else if (document.layers) { document.layers[name].innerHTML = http.responseText; alert (10);}
 }
 alert (11);
}
Line 28, Char 4 apparantly. As you can see by all the alerts, I'm doing a lot of debugging.

See my code live at http://ads.imywebsite.com/adtest2.php

I've been messing with this for a couple days already...