you need to look in the file inc_header.asp for the following code (it should be on line 280 in an unmodified file)
<a href=""default.asp"" tabindex=""-1"">" & getCurrentIcon(strTitleImage & "||",strForumTitle,"") & "</a>
if you change it to
<a href=""" & strHomeURL & """ tabindex=""-1"">" & getCurrentIcon(strTitleImage & "||",strForumTitle,"") & "</a>
that should do the trick