Custom 404's can be set for ASP and .NET sites by adding the following code to the bottom of the web.config file before the final </configuration>. You will need both options to redirect all 404's.

Classic ASP and Static Content

<configuration>
  <system.webServer>
         <httpErrors errorMode="DetailedLocalOnly" defaultPath="/404.asp" defaultResponseMode="ExecuteURL">

            <remove statusCode="404" subStatusCode="-1" />
            <error statusCode="404" prefixLanguageFilePath="" path="/404.html" responseMode="ExecuteURL" />

         </httpErrors>
  </system.webServer>
</configuration>

.NET

<configuration>
  <system.web>

    <customErrors defaultRedirect="404.aspx" mode="DetailedLocalOnly">
      <error statusCode="404" redirect="404.aspx"/>
    </customErrors>

  </system.web>
</configuration>

This exampled shows 404's for all content redirecting to index.html:

<configuration>
  <system.webServer>
         <httpErrors errorMode="DetailedLocalOnly" defaultPath="/index.html" defaultResponseMode="ExecuteURL">

            <remove statusCode="404" subStatusCode="-1" />
            <error statusCode="404" prefixLanguageFilePath="" path="/index.html" responseMode="ExecuteURL" />

         </httpErrors>
  </system.webServer>
     <system.web>
                 <customErrors mode="On">
                     <error statusCode="404" redirect="/index.html" />

                </customErrors>
     </system.web>
</configuration>