you can connect to a VPN server from inside the network. That is how i did some testing of my VPN server on my network was to first connect from the inside.
You should check out Microsoft's Knowledge base to find out how it is done if you are unsure. I have not done a VPN connection with XP home has the server. But XP should have wizard to help you and open the proper ports to allow client connections.
Out of curiousity, why are you doing it this way? What are you trying to accomplish? I am just curious.