> A curiosity to me.
> 
> On  a forum I have seen the forum can be accessed
> by:
> 
> www.websitename.com 
> 
> or just 
> 
> websitename.com (leaving out the www.)

Those two are different sites or can be. 
Usually both sites if they exist are the same machine so you get the same
result. 
Sometimes the without www will autoforward to the www machine. (try google.com
and look at the browser's url bar)
Some browser's when they get an error tries the www for you, some do not.
and sometimes you get an error message without the www.

Httrack isn't a browser and since they CAN be different sites, you must put
the proper url.