Setup OpenVPN

I need to setup OpenVPN so that a remote computer can connect to internally networked windows machine to connect to a service. Currently I am using the service with static IPs at my residence, but it's not secure. Anyone can utilize the service if they know they IP and port. I need to make sure it is secure with OpenVPN. Set it up so that a remote machine can have OpenVPN client installed, and use a key to connect to the VPN, thereby gaining access to the internally networked machines. I have an unmanaged switch available for this, and a total of 3 (should require 2) NICs on the server for OpenVPN. It's running linux, the rest of machine on both ends will be windows. I tried a few guides, and was only able to establish a tunnel between two linux boxes. I could not establish a connection between the internal network and a remote machine. This should be easy for someone who has the experience.