Flash "sees" paths relatively to the page it is on.
So while it's location might be /Portals/0, it uses the path in the address bar (of the page).
On the home page that is you domainname, but on subpages it will use
www.mysite.com/about/contact/tabid/122/default.aspx, while these "folders" do not exist.
So if you reference test.xml it will look for:
www.mysite.com/about/contact/tabid/122/test.xml
This means you will have pass the actual path to the SWF somehow.
You can pass it to the flash movie (as a variable) from a module, there are some free Flash mosules, that allow you to pass a variable to the flash movie.
I mostly use the one from http://www.dignuke.com/ for this