At the moment I have my Windows desktop computer at home, it's always on and always online. I want to be able to leave it there and then be able to remote desktop to it from my laptop from a remote location. I only want my laptop to be able to access it through remote desktop though so could I do this with some sort of mac address filtering or is there another very secure way to do this, possibly keys?
Setup a firewall and run the RDP port on a custom port. Set the firewall to only accept certain IP ranges. Additionally you could setup two-factor authentication. I use this on a few servers myself. Checkout https://www.duosecurity.com/microsoft and https://www.duosecurity.com/docs/rdp
You won't be able to do it via Mac Address unless your laptop is connected directly on the same network - that's not how networks work. IP address filters will work assuming that that are always connecting via the same IP each time which seems unlikley from what you are saying. The next best thing then is remote access software from someone like TeamViewer (http://www.teamviewer.com/en/index.aspx) that will allow you to access your PC securely from any remote location. You will then probably be able to setup a firewall rule restricting RDP access to certain IPs belonging to teamviewer. I know a lot of PC maintenance companies who use this software for remote access to client systems and they speak very highly of it. This would probably be your best bet, but it isn't cheap.