Joerg is right. There's 3 ways off the top of my head to initiate this from the outside:
1) Corporate Service
Services like GoToMyPC and LogMeIn (has a free version) were designed to handle this. But with this, comes monthly fees and it's not a full connection, just an RDP simulant.
2) Hamachi
Create your own hamachi network, and you'll be able to connect to it as if it's on the LAN (works excellent when you also want to host LAN parties and not worry about port forwarding / a dedicated server). Probably the easiest, and free, but you'll need to install hamachi, so if you're using public terminals that you don't have admin rights on, you're SOL.
3) Make your own ReverseSSH/VNC Connection
Two ways i can think of doing this:
a ) Having a dedicated/virtual private server to forward this to (pricey, but you can do so much more with the VPS and you'll have a public IP presence) - You could always find a friend and bum a port and some bandwidth off of him.
b ) Hack it - Setup a Cronjob or Scheduled Task to phone home to a website constantly, pull a file that gives an IP and port, and reverse tunnel to that IP/Port. Make sure you check the public keys if you do attempt to do this to prevent any kind of hijacking attempt, and this won't get you arround a firewall if you're behind one on your side.
My personal recommendation is do number two if you're trying to do it quick, easy, free, and 3a if you're looking to have some fun and have the most felxibility - just watch your bandwidth usage over that link.