When using an NT (or 2000) OS, one might encounter problems routing Oracle access through the firewall, since the default communication is NOT done using ONLY one port.
anyway: A LOT of pages on the web suggest adding "USE_SHARED_SOCKETS" in the Oracle registry entry (HKLM-LocalMachine/Software/Oracle), but in fact it should be "USE_SHARED_SOCKET" (no 's' at the end - no plural)
...cost us a lot of time and nerves to find this tiny little bug.