That depends on what client you are using for the VPN tunnel. What you are looking for is called split tunneling, look it up on google and it should tell you have to do it depending on your client. You are also going to have to put in static routes into your PC (I’m assuming that is your PPTP client) so it knows how to reach the private subnets on the router. You should be able to do this as part of the PPTP tunnel negotiation.
Your Windows machine itself needs to know how to get to your remote subnet. Without that route, it will not work. That is because without a specific route for that subnet, Windows will just use the default route it has. There is nothing the MikroTik can do about that.