Since IIS 6, Microsoft has made it very easy to troubleshoot the cause of a 404 "not found" error from the web server. Most admins don't realize that the IIS log file can now include the substatus code along with the 404. The first step
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Read the complete post at http://blogs.iis.net/tomkmvp/archive/2009/04/27/troubleshooting-a-404.aspx