Can't browse a website http error 500.19 - internal server error

I need to make some code changes to a website, except I can't find the code in any source code repository. The code only seems to exist on the IIS server itself. I have logged onto the IIS server that hosts the files and copied all the file contents to my local machine. I have opened the website from my machine in Visual Studio and it builds fine. However, when I try to 'play' the solution in Visual Studio it opens up an explorer page with the error 'http error 500.19 internal server error. Error code 0x80070021. The configuration section cannot be used at this path. This happens when the section is locked at the parent level. Config source is < Windowsautentcation enabled="true">. I can't get to the stage of starting my code changes until I can 'play' the existing code successfully. Any ideas?