> However one of the forums I would like to mirror
> requires a login to see all the topic or to download
> the attachments. 
> (not http authentication but a login form)
> 
> Is there a way to automatically fill in a login form
> or to somehow use an existing cookie (from IE or
> firefox) to allow it to regognize my username?

Try this
<http://forum.httrack.com/readmsg/10597/index.html?pid=10577>

but set your start URL to a forum page that has one of the login forms.