I'm using Windows XP Pro, configured to access internet via
LAN via an automatic configurations script (i.e. a *.pac
file).  
IE: Tools --> Internet Options --> LAN Settings
MOZILLA: tools --> Options --> general --> Connection Settings

Problem:
Httrack does not allow me to put the *.pac in the proxy
preferences, and there is no actual 'proxy server' running
on the ip address where the *.pac file is located.  As it
is, the *.pac file involves scripting of some sort, and
httrack can not interpret it.

Solution:
I used IE to to look at the *.pac file.  Several times in
this file, I see lines of the format:

isInNet(myIpAddress(), "xx.xx.xx.xx", "255.255.255.0") ||
isInNet(myIpAddress(), "xx.xx.xx.xx", "255.255.255.0") ||
....
return "PROXY xxx.xxx.xxx.xxx:nnnn";

Depending on which 'myIpAddress' matches my current ip
address (I get this from running ipconfig /all in the
command line), the script executes the appropriate return
"PROXY... line.

This PROXY (and port number) is the one to supply to Httrack
 in the proxy tab of preferences.