The website declined to show this webpageNow, there are a few causes to this problem. The most common is when the user forgets to add an index page to the site or, to add the index page to the list of "Enable default content page" under IIS.
Mr. Ian Tinsley found a more sinister cause that has the following symptom: if you browse to an existing .aspx file, ISS will return a 404 error code, even though the file exists. A good way to check if this is your case is to place a static file (htm or gif etc) in your site and try browsing it; if it shows you the static file then you probably have a fake 403/404 scenario.
To make sure, go to Web Service Extensions under IIS and check if ASP.NET v2.x is listed:
The tricky bit is: even if ASP.NET v2.x is not listed above, the v2.x extension still shows on the application properties. So if you don't have it above, you need to run aspnet_regiis.exe:
Running with -ir will help keep your asp.net v1.1 stuff running and still install v2: