Missing module

You should check your http.conf, and scroll all the way down to the bottom. If the installer got PHP installed correctly, there should be an entry for PHP, like shown below.

#BEGIN PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
PHPIniDir "C:\PHP"
LoadModule php5_module "C:\PHP\php5apache2_2.dll"
#END PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL

If for some reason this is missing, such as the retarded permission system on Windows (or whatever its called), then add it manually. The file paths should reflect your own installation.

Empty module paths

You might also (for whatever reason), experience that the paths are left empty by the Installer, in which case you should fill them out on your own to solve the problem.

#BEGIN PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL
PHPIniDir ""
LoadModule php5_module ""
#END PHP INSTALLER EDITS - REMOVE ONLY ON UNINSTALL