HTTP 404.3 Error for IIS 7 / ASP.Net Website

If you get the following error when accessing or trying to access a .ASPX page then you just need to install the ASP.NET extensions in IIS 7. 

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.
 
See article: Installing IIS 7.5 in Windows 7 for how to install the extensions.