why dont you try like this

1) go to internet and open the main site
2) enter login id and passwd
3) come to next page after login

copy this url (this might be having some session id or cookies will be at ur
machine) and paste this url in httrack and download the files now

** dont close the browser while mirroring with httrack as it may effect ur
current session id whch u r using in httrack


just give a try ...