The usual way to do this is via network sockets. The observers would connect to a network port on the subject PC, and the subject would pass notifications to all observers.
Since there's no standard C++ socket API, so how to code it exactly depends on your platform, but pretty much every OS has a variant of the Berkerley socket. There's a good introduction to it here:
http://www.troubleshooters.com/codecorn/sockets/[
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The next problem is how to transfer data from subject to observers, since you can't send pointers to areas of shared memory over the network. For this, you'll have to come up with a suitable protocol, which would normally be text-based.