A few things to check are permissions on the PHP installation folder and also make sure the mapping is correct. For permissions checking you could run this program http://www.sysinternals.com/Utilities/Filemon.html and run a PHP page and then stop the file monitor. Once you stop the monitor, scroll down and look for any problems that might exist.

From my understanding, PHP 5 isnt always compatible with things written for PHP 4 so you need to be careful there and see if there is phpBB available for PHP 5.

Let us know how these tests go.