1. you are trying to access extensions which are not registered in IIS or webserver.
To register your custom extensions open web server and add MIME type with appropriate type.

2.  you installed the IIS 6.0 and framework 2.0 on windows 2003 operating system. After deploying the web you observed that web is
responding an error with 404.
  By default, once you installed the IIS. It want not allow to run ASP.net or web pages or CGI, perl etc.
you need to allow them to fix this error. You need to open IIS 6.0 and find the node called Web services extensions.
In right pen you can allow or prohibit the access to ASP.net 2.0 or CGI.
For more information you can refer below link
http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/44f16c37-f727-4244-9813-2289e13dadba.mspx?mfr=true

or you can use support forum at www.iis.net

The default IIS settings will allow you to only see the Static pages.

Another reason for file or directory not found is, you don't have configured correct runtime for asp.net web. To configure
asp.net runtime you need to execute aspnet_regiis found at appropriate version of .net framework.
For 64 bit machine look at Framework64 folder and for 32 bit look at Framework folder.

refer http://www.revenmerchantservices.com/post/2010/07/30/Removing-aspnet-runtime-from-IIS.aspx

Satalaj