AJAX, Eval(), And Javascript Within Javascript

Ok so, this is my purpose:
- to be able to load asynchronously (via AJAX) some javascript ads (like google's or adbrite) so as to make them be loaded in the background, then update the page after the ads have loaded via innerHTML

Why?
-Because 90% of the time in my newer sites, javascript ads are the major offender in terms of speed of page rendering

My problem:
Via ajax, I can call a php file that retrieves some javascript and outputs it, XMLhttprequest returns those javascript lines, but they don't render in the page, since they miss the whole page loading, and are apparently not parsed
For example, let's say I call a php file via ajax, and it returns the output into a variable named "text" containing "document.write('hello')"
if I use xxx.innerHTML=text, nothing happens

My 1st solution:
Passing those javascript lines to eval() [like eval(text) ], but this produces a second problem, that I couldn't solve (probably because of my lack of knowledge in javascipt):
if I eval the code, it deletes my current page and renders a new one
for example, if I parse a document.write, my page disappears, and a new one is rendered with the document.write text

What I want is basically to make that "document.write" appear inside a div in my page, adding to the content (and not overwriting the whole page), much like what happens when using innerHTML

Is this even possible? How would you go about it?
I tried xxx.innerHTML=eval(outputfromphpfile) but it overwrites my whole page...