Hello Mark,

The short answer is you can do this several ways; through DNN or add a custom 404 to the root of your DNN or cahange the IIS default 404.

To change it in DNN login in as Admin or Host and got to Admin > Languages, there select Language Editor. This will open the GlobalResources.resx file. The 404.error is the second item down. This will only work if you have select to "Use Custom Error Messages" on the Host > Host Settings tab.

To add a custom 404 or other error messages to your root (my prefered method), simple create a new 404 page and change your web.config files customErrors mode from this:

<customErrors mode="RemoteOnly" />

to something like this:

    <customErrors defaultRedirect="ErrorPage.aspx" mode="On">
      <error statusCode="500" redirect="servererror.aspx" />
      <error statusCode="404" redirect="filenotfound.aspx" />
      <error statusCode="403" redirect="AccessDenied.aspx" />
    </customErrors>

Hope this gets you started.