XP’s VPN through Cisco 3640 Router

We have set up a VPN server in a XP Professional PC. Another XP’s VPN client can make a connection to the VPN server successfully when the client PC is in the same subnet with the outside-interface of the VPN server. But the VPN clinet cannot make aconnection to the VPN server if it passed through a Cisco 3640 router from another subnet although the client PC can ping and traceroute the VPN server’s outside interface through the router. Seems not a routing issue. Do we need to do any thing in the router? The router loads with IOS 12.2 IP only and it doesn’t have any access list to limit the traffic.

Wei