any good and simple RPC library for inter-process calls?

You might like ZeroMQ for something like this. Perhaps not as much a complete RPC, as a raw byte messaging framework you could use to make an RPC. It's simple, lightweight and with an impressive performance. You can easilly implement an RPC on top of it. Here's an example server straight from the manual:

//
//  Hello World server in C++
//  Binds REP socket to tcp://*:5555
//  Expects "Hello" from client, replies with "World"
//
#include <zmq.hpp>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int main () {
    //  Prepare our context and socket
    zmq::context_t context (1);
    zmq::socket_t socket (context, ZMQ_REP);
    socket.bind ("tcp://*:5555");

    while (true) {
        zmq::message_t request;

        //  Wait for next request from client
        socket.recv (&request);
        printf ("Received Hello");

        //  Do some 'work'
        sleep (1);

        //  Send reply back to client
        zmq::message_t reply (5);
        memcpy ((void *) reply.data (), "World", 5);
        socket.send (reply);
    }
    return 0;
}

This example uses tcp://*.5555, but uses more efficient IPC techniques if you use:

socket.bind("ipc://route.to.ipc");

or even faster inter thread protocol:

socket.bind("inproc://path.for.client.to.connect");

You probably don't even need a library. Windows has an IPC mechanism built deeply into its core APIs (windows.h). You can basically post a windows message into the message-queue of a different processes main window. Windows even defines a standard message to do just that: WM_COPYDATA.

• MSDN docu on WM_COPYDATA
• MSDN demo code
• More demo code the following StackOverflow response

The sending process basically does:

• FindWindow
• SendMessage

The receiving process (window):

• On Vista and later has to modify its message filter using ChangeWindowsMessageEx
• Override its WindowProc
• In order to handle the incoming WM_COPYDATA