I have two computers connected with VPN, and some virtual machines on each. I want everything to see each other (that is basically 4 different networked machines). What I've not managed so far is that a computer on the one end to be visible to the other end. In tracert terms:

192.168.78.42>tracert 192.168.69.18
Tracing route to WIN-2K8R2 [192.168.69.18]
over a maximum of 30 hops:
  1    <1 ms    <1 ms    <1 ms  192.168.78.17
  2     *        *        *     Request timed out.
  3   217 ms    78 ms    78 ms  WIN-2K8R2 [192.168.69.18]
Trace complete.
192.168.78.42>tracert 192.168.69.112
Tracing route to 192.168.69.112 over a maximum of 30 hops
  1    <1 ms    <1 ms    <1 ms  192.168.78.17
  2   333 ms     *      337 ms  WIN-2K8R2 [192.168.86.22]
  3/4/5     *        *        *     Request timed out.
  6  ^C
192.168.69.18>tracert 192.168.69.112
Tracing route to 192.168.69.112 over a maximum of 30 hops
  1    <1 ms    <1 ms    <1 ms  192.168.69.112
Trace complete.
192.168.69.112>tracert 192.168.78.42
Tracing route to 192.168.78.42 over a maximum of 30 hops
  1     1 ms     *       <1 ms  192.168.69.18
  2    79 ms    77 ms    80 ms  192.168.86.21
  3    80 ms    77 ms    81 ms  192.168.78.42
Trace complete.

Note: the 4 machines are 192.168.69.112 (winXP), 192.168.69.18=192.168.86.22 (win2K8R2), 192.168.86.21=192.168.78.17 (Linux), 192.168.78.42 (win2K3).
The VPN is a TAP openvpn connection between 192.168.86.21 and 192.168.86.22.
I would say that the problem is in the win2K8 machine, but Windows networking is my weak point.