Can I make this network and keep the Windows XP computer protected from the internet at all times?
What specifically do you mean by this?
Do you mean you do not want the XP machine to be able to connect to the Internet at all?
Or do you mean that you don't mind it being connected to the Internet, but want some sort of protection from the Internet (i.e. firewall)?
If you mean the former, then I believe the double network card on the Windows 7 computer that CaveDweller2
should do that assuming you don't turn on Internet connection sharing. I assume this would work, but don't have any direct experience with this approach.
If you mean the latter, then just connect the XP machine to the router. The router will act as a firewall and in general should be enough protection for incoming traffic, but if you want to be extra safe, then install a software firewall on the XP computer. The XP computer would still be able to "go out to" the Internet. You could potentially prevent this by configuring the software firewall to prevent outgoing Internet connections, but allow connections to the Windows 7 computer…depending on the software firewall that you use.