For some time my Visual Studio has had a problem displaying web pages when debugging, and if you've been keeping your windows updates running, most likely you've got the same problem.

This happens when you're using the built-in web server and Visual Studio uses http:localhost: /site. In fact, any reference to localhost fails to find the page you're looking for.

Turns out that an update to Windows Defender messed around with the hosts file which lists host names and matches them to predefined IP addresses. This file resides in C:\Windows\System32\drivers\etc (on Vista at least) and you'll find the following line inside:

::1             localhost

It should be:

127.0.0.1 localhost

Problem solved!