Post-scriptum: The suggested system will work without 
troubles the first time ; but remember that if you also 
want to update the website, by fetching only new material, 
you'll have to handle existing files and new ones:

- when checking "receive-header", ignore 302 responses 
("not modified") and keep the existing file already 
generated in the arc file

- when handling "transfer-status", update old data if 
necessary

Also, remember that httrack sometimes makes TWO requests 
for a single file, to pre-check the file type (EHAD 
request). This will not be a problem if you store the 
headers in a table, and overwrite the second one (which 
will always be THE get request)