VPN issue

Hi there.
I have a vpn between two pc, one as server, the other as client. Router, as
well as windows firewall of the server are configured to let the vpn work
(port 1723 and GRE protocol). In fact, when the client calls the server, the
vpn is well established and the client is well authenticated.
Now, the point is, the client can't access the shared folders of the server.
But if i disable the server windows firewall, then the client gets access to
the shared folders perfectly. How come? What do i need to
open/enable/disable in server windows firewall in orther to let vpn full
working, but keeping firewall enabled?
Thanks in advance.
Pablo.