ASP.NET IIS Registration Tool (aspnet_regiis.exe)

Let's say you install a new version of Visual Studio which also installs a new version of the .NET Framework.  If you are using IIS (not IIS Express) on your local PC to host a site you are developing, it won't work until you register the new version of the framework with IIS by using the registration tool.  The tool exists in each version of the frameworks folder.  Just browse to the folder of the framework you want to register and run the tool with the -i option.

Open a Command Prompt as an administrator.

cd C:\Windows\Microsoft.NET\Framework64\v4.0.30319 for example.

Then, aspnet_regiis.exe -i

MS info on tool