urgent!! add coding to every page

[CYBERWORLD]

I host some of my friends sites on my server

I would like to add ad banner or pop up windows to every page I host for them, just like what geocities.com are doing

But I don't want to add the code to every page on my own.

Can I change the server setting such as .htacess or whatsoever to do this? such as to parse every .html document to add the code at the beginning of the page
Or, I prefer to do it with cgi script so that I don't have to upload the pages again after edit....

Please help!!