The quickest way is to use bridge mode. But bridge mode will generate a lot of traffic. To use routed mode, you have to use unix based system for either client or server and read the
http://openvpn.net/index.php/open-source/faq/77-server/321-remap-local-addresses-to-connect-two-networks-with-an-overlap-in-the-private-address-range.html