I'm a little unsure whether this is a pure PHP problem, but I'm a little lost, so please bare with me.

I'm running an Apache server (2.2.16) on Windows XP installed from the standard win32 binary, with PHP 5.3.3 (again 32bit standard win-installer from php.net).
Almost all settings (in both Apache and PHP) are the default ones.

The problem: I have a web app that uses only 1 page but a bunch of AJAX calls to the...