Installation of ASP.NET MVC 3 or ASP.NET Web Pages fails with a fatal error

A fatal error message similar to the following occurs when you attempt to install ASP.NET MVC 3 or ASP.NET Web Pages:





To determine the exact failure that occurred, click the log file link on the last screen of the installation process for ASP.NET Web Pages or ASP.NET MVC 3. Locate the error code that made the installation fail. This error in turn points to the MSI log, which is located in the same folder. The MSI log will report an error similar to the following:

This error condition can be caused by the following circumstances:

• You have a pre-release version of ASP.NET Web Pages or ASP.NET MVC 3 installed and you installed Visual Studio 2010 SP1. When you install the released version of ASP.NET Web Pages or ASP.NET MVC 3, you receive the error listed above when the installation fails.
• You have sections in the root Web.config file marked with allowOverride="false". (The root configuration file is in the %system%\Microsoft.NET\Framework\<version>\Config folder).
• Another process has locked the root Web.config file. This prevents the installer from writing to it.
• The root Web.config is read-only.
  

The solution depends on which of the possible error conditions occurred. If you have a pre-release version of ASP.NET Web Pages or ASP.NET MVC installed, try the following:

1. Remove the trailing backslash from the following registry keys:
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\4.0.30319.0\Path
   HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\ASP.NET\4.0.30319.0\Path
   
2. Uninstall the old version of Microsoft ASP.NET Web Pages.
3. Add the trailing backslash back to the registry keys.
4. Install ASP.NET Web Pages or ASP.NET MVC 3 again.
   

Otherwise, try these steps:

1. Change any sections in the root Web.config file that have allowOverride="false" to allowOverride="true".
2. Determine what process is locking the root Web.config file and end that process, or restart the computer you are trying to install to.
3. Make sure that the root Web.config file is not set to read-only.
4. After making these changes, install ASP.NET Web Pages or ASP.NET MVC 3 again.
   

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