Unfortunately, because you are running a build of Windows Vista that was
released only to a very select group of entities, you will probably not find
a colleague here with the same build.
However, I can confirm that in build 5472 while logged into the guest
account, the network icon in the system tray shows a red X and "access
denied" in the tooltip. But, this does not stop me from accessing my local
network or the internet.
I assume this X just indicates that the guest account does not have
permission to view or modify network settings.
Are you sure you have set up the network connection correctly while logged
in as an administrator? For examle, if you are NOT connected to a domain,
you should have set your network connection to Private Network instead of
the default of public network.
You can change this setting from the Network and Sharing Center in the
Vista Support FAQ http://www.jimmah.com/vista/