IIS express applicationhost.config security is reset every time a solutions is opened in VS2012

I have an MVC4 application been developed in VS2012 that uses forms authentication.

[authentication mode="Forms"]  [forms loginUrl="~/Account/Login" timeout="2880" /][/authentication]

It's using the default IIS 8.0 Express.

Every time I open and run the app from within the VS I got the message:

HTTP Error 401.2 - Unauthorized

You are not authorized to view this page due to invalid authentication headers.

I found out that to avoid this screen I have to change to 'true' the anonymousAuthentication and windowsAuthentication keys of the Security section of the applicationhost.config file at the [User Documents]\IISExpress\config folder to:

[system.webServer]  [security]    [authentication]      [anonymousAuthentication enabled="true" /]      [windowsAuthentication enabled="true" /]    [/authentication]  [/security][/system.webServer]

But every time I reopen the solution in VS the keys are reset to false.

How can I make then permanently set to true?

I've tried to put those key in the application's web.config but then I receive the message:

HTTP Error 500.19 - Internal Server Error

The requested page cannot be accessed because the related configuration data for the page is invalid.

Any help folks?