> Hmm, I change within the IIS the custom Errors property of 
> HTTP Error 404 from type 'file' to 'url' and send it to 
one 
> of my ASP pages '/error/eshow.asp?error_id=16&'. This is a 
> page that has the layout (including graphics) of the site 
> and it's working without any problem. I'm not sure whether 
> this creates a 200 page or still keeps the 404 

For one of our IIS websites we use a custom 404 error page 
pointing to '/404.asp'

While HTTracking the site one day, I was very surprised to 
discover that IIS was actually returning a 200 response for 
this error page!

A very easy solution to this is to enforce the ASP to 
respond correctly as 404, but adding the following above any 
HTML output:
<%
Response.Status = "404 Not Found"
%>

Also, you can check what response the error page is giving.  
Create a test HTTrack project and set the start URL directly 
to your error page (/error/eshow.asp?error_id=16) or a 
non-existant file (/doesNotExist.htm), run the mirror, then 
look at hts-log.txt

12:00:00	Error: 	"File Not Found" (404) at link www.
foo.com.au/doesNotExist.htm (from primary/primary)

and you will see the response code there.