Remote Desktop is included with all Windows computers, although if you remote into a non Server version of Windows with remote desktop the local user cannot use the computer while you are remoted in, with Windows Server you can remote in without interrupting the local user.
Please note not all versions of Windows can be used as a remote desktop server (the computer you are connecting TO), for instance the only version of Windows 7 that can be a remote desktop server is Windows 7 Professional. All versions of Windows can be used as a Remote Desktop Workstation or Client (the computer you are connecting from).
The link below explains how to configure remote desktop, or you could just search "Configure Remote Desktop".
The link below explains how to configure your router for use with remote desktop, or search "configure router for remote desktop".