ERROR 500 - Internal server error on PHP Sites

I am running IIS7.5 on Windows Server 2008 R2 SP1. Over a month ago I did the generic installation of PHP (v5.5.11), MySQL and the phpMyAdmin interface and it all worked great. The other day I started playing with WordPress sites and it all got ugly. Now on any site that involves PHP I get either the "500 - Internal server error" (including phpMyAdmin) or the browser just sits there forever "loading" but not receiving any data as if it is still trying to resolve the URL.

I have tried installing the 32-bit version of the Visual C++ Redistributable for Visual Studio 2012 Update 4 as suggested here: 500 error on fresh install of IIS and PHP

I have been tearing my hair out all day. Any help would be much appreciated!