Just installed 4.9.2 on my Vista box, and got a basic portal up and running locally. When I try to look at it from my laptop on my internal network, I can see static image files in my folder http://vistabox/portal/ , I can see ErrorPage.aspx correctly (which tells me IIS and ASP.NET are running properly), but trying to navigate to http://vistabox/portal/Default.aspx , I get bounced to http://localhost/portal/ which shows "Page cannot be found".

Just to add insult to injury, I went back to the Vista machine, tried to log into the portal locally, and user "admin" now no longer has admin rights ?!? Unlikely to  be someone coming in from outside messing with it, as I installed with a non-default password on both "host" and "admin"

No, same problem - log in as host or admin, and all I can see is the home page (not the forum etc.) with no admin options.