Hi there
I'm running Drupal 6 on IIS7/php5.3 - I get the 500 error on /admin/build/modules 'save configuration' even though changes I've made are saved - I also get the same error when I select 'flush all caches' from the admin menu module (turns out I get a 403 when I flush the 'theme' cache - so that must be the problem - I am using a custom theme - a subtheme of blueprint which Ive created).
The latter problem suggests a permsissions error (i do not have direct access to the server) but I wondered if the first problem is something IIS7 specific?