Setup a VPN and then you can RDP to any windows computer on your network. Added benefit is that it's hundreds of times more secure than simply leaving a port open or running TeamViewer.
OpenVPN is free and runs a few routers out of the box. Most others you can load dd-wrt or tomato onto. Or you can be an uber-nerd like me and run a software router like PFSense that supports OpenVPN.