SBS2003 VPN router-to-router problem.

[pccorrect]

I need help adding a remote pc to an SBS2003 domain over VPN.
Here are the details:
:
Main Office: Vigor2600V router server-SBS2003. 2 Network Cards. Firewall Disabled
(Server)
Server's LAN NIC: 192.168.16.10
Server's WAN NIC: 192.168.1.2

Router's LAN Port: 192.168.1.1
Router's WAN Port: assigned dynamic IP from ISP

Branch Office: Vigor2600V router-Windows XP Professional-Firewall Disabled (NO Server) Workstation's LAN NIC: 192.168.5.10

Router's LAN Port: 192.168.5.1
Router's WAN Port: Fixed IP from ISP

Current status: We have a 'L2TP 3DES-MD5 Auth' LAN to LAN vpn connection running between the Main Office and the Branch Office ROUTERS. We can ping from branch office to: 192.168.1.1 and 192.168.1.2 and We can VNC (the main office's server) from branch office to 192.168.1.2

Problem:
----------
We CANNOT ping SBS Server - 192.168.16.10
We CANNOT ping other pc's on the SBS LAN (192.168.16.101,192.168.16.102...)
We cannot join the remote pc to the domain.

-DHCP on server is enabled.
-Routing and Remote Access on server is enabled (If we disable it we can ping 192.168.16.10 but LAN users loose Internet connection!).
-DNS is enabled.
-WINS is enabled.

ANy ideas ??