Sorry I answer so late. I took a good look at your files and I tested them in firefox on my old dying server.
Unfortunately wherever I choose to include the one file you posted (there are four places where you <? include ?> something) I did not get the wanted behavior (horizontal scrollbars).
So I have to assume the problem might be with the css file (css.php) or that the big file you posted is only the code for a frame (you did say the site uses frames).
For a further easy test I suggest you disable the css code in your borwser (it's really easy if you have firefox and
this extension which I really like). Good luck, and let us know about your results.