I you are behind a firewall and you have no access to change the port forwarding settings (like behind a college firewall) then try some of these site.

LogMeIn
Webex
GotoMyPc

These site will have the host computer connect to their servers through the internet. The remote user then logins into the website and is able to communicate to the host computer. Some of the services are free and some you have to pay for. I have never used any of the services, but others I have talked to say they work just fine.