HTTP Error 404.3 – Not FoundThe 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.

I was getting the above error while trying to run an ASP.NET page deployed inside IIS 8 in Windows 8 machine.

The fix was to enable the ASP.NET option of the IIS feature

Start -> Run -> appwiz.cpl -> Turn Windows features on or off -> Internet Information Services -> World Wide Web Services -> Application Development Features

Hope it helps.