Blank Page when product added to category

I'm using 1.3.8a and had all products in and working.
But we had to remove and rename some categories and products and then when I created the new category it showed up all right.

BUT when I added a new product to that category it only returned a blank page.

Reading the forum I downloaded the myDEBUG utility and ran it and got this:

[10-Sep-2010 02:27:09] PHP Warning: Cannot modify header information - headers already sent by

(output started at /home/mtm/www/mtm/includes/modules/pages/index/header_php.php:7) in

/home/mtm/www/mtm/includes/functions/functions_general.php on line 44

Now I wonder how to fix this problem?
Thanks Roger
The site is: www monthly tax mentor .com and the link that doesnt work is the "Tax Talk Archives" on the nav bar.

OK Already solved this! So for the others out there heres what I found.
The myDEBUG utility http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_54&products_id=860 was really helpfull in locating the possible corrupt files. Though I didnt get a lot of the code, I did notice in the includes/modules/pages/index/header_php.php file that there was about 4 lines of blank white space at the top of the page. While I didnt really think it would fix it i remembered that that could be a problem, so I backspaced the code to the beginning of the first line. Amazingly enough that repaired the whole thing!

One other thing...I thought that it was still not finding the categories properly as it seemed to go straight to the product, UNTIL I read the code more carefully and found that IF there is only one product installed the default is to bypass the category and go straight to the product. (you can change that by changing the true/false setting)
Rog