I think I know what's causing this. It's possible that you're having a problem with BOM (Byte Order Marks) encoding. If you view your site with Firefox, you'll notice a couple of foreign characters are output to the browser before the rest of the page. These characters are used to identify the UTF encoding that's being used. You need to remove these characters by saving your pages without BOM encoding.
This would explain why the error messages were suggesting that there's a problem with both index.php and sessions.php. Data is being sent to the browser before the start of every page.
You can read more about this problem on
http://uk3.php.net/manual/en/functio...tart.php#68418
There's other posts about it on that page, if you do text search for "UTF" you'll find them.
HTH