The tcp port 1723 is used by PPTP VPN protocol but I had some problems and I removed it from ZeroShell. I will try to support it as soon as possible. At moment the only supported VPN host-to-lan is L2TP/IPSec which is much more secure than PPTP protocol. Windows XP has a builtin client for L2TP/IP and you could use that. The only problem is that you have to generate a host X.509 certificate and install it in the computer account. This Certificate with its private key are used to authenticate both IPSec endpoint (client and server). To create the host certificate you could use the ZeroShell Certification Authority. The user is then authenticated with MSChapv2 with the same username and password used on Kerberos 5 KDC.
Your LAN-to-LAN VPN setup is very interesting. Could you describe it using pdf or html format?
If you want I will link it from the ZeroShell documentation page.