I paid my "Windows tithing" recently and did a complete reinstall. Fortunately Windows is now a guest VM inside of a Linux host. A settings change I had made a long time ago but forgot to reapply during my reinstall was for IIS. Whenever I was developing--even locally--I would get "500" errors from IIS which would then display a generic and very unhelpful error page.
The solution is to go into IIS and disable generic error messages:
http://mvolo.com/blogs/serverside/archive/2007/07/26/Troubleshoot-IIS7-errors-like-a-pro.aspx