ASP is a server side language that serves html pages.
HTTrack downloads the html produced and modifies the extensions in the pages.
If you want to browse the site offline, index.html should be the offline copy
of index.asp or any default file set on the server. (You can get the ASP
source if you have the webmaster's user name and password.)
If your file index.html is empty or useless, it may be because the server
blocks you as you are trying to mirror the site. As you modified the
robots.txt option, you can also modify the browser ID so that you are not
blocked on that basis.
Limit the bandwith usage and number of connections so that you  and the next
users are not blocked on that basis too.