IIS error messages are fairly meaningless. I'd first suspect a coding error (which means one of their guesses might be on target).
If you have php.exe in your path, you can do something like:
c:\websites\somewhere > php -l index.php
Which might tell you if there are errors. Unfortunately, my Windows PHP installation doesn't log them anyplace I'm aware of. You might try reading system logs or asking one of the very smart search engines about it, though.
