Here Mudassar Ahmed Khan has explained how to register ASP.Net with IIS i.e. installing the ASP.Net in IIS so that ASP.Net pages can be served from IIS server. If ASP.Net is not registered with IIS then when accessing ASP.Net Website you will get the following error.
HTTP Error 404.3 - Not Found
The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.
Once the ASP.Net is registered the IIS will show ASP.Net 4.0 Application pools.