" You don't mention your OS. If it is Vista, an automatic update or certain virus protection apps may have made a change to your 'hosts' file.

Your "hosts" file is what tells your computer that "localhost" should map to "127.0.0.1" so here is where your file is:

    C:\WINDOWS\system32\drivers\etc\hosts

Close Visual Web Developer and any browser that open.

Locate the 'hosts' file using Windows Explorer

Open that file in Notepad under a 'computer administrator account'. The following entry should be present. If it is not, or does not look like the following, add or edit the entry:

    127.0.0.1       localhost

Save the file and reboot your machine.

Try to open the page(s) that were giving you the 404 error. "

http://forums.asp.net/t/1437368.aspx