If you have no control over the network where the machine is located that you want to control, you will need to use a service like gotomeeting.com. This will allow you to remote control a machine via port 80, but you will need to install some software and have someone on the other end to accept.
If you can open port 3389 tcp on your firewall inbound or you are on the same network as the computer you want to control, then you can use Microsoft’s remote desktop client. All you need to do is right click on my computer on the computer you want to connect to. Then you choose remote on the top tab and make sure the check box is selected next to “allow users to connect remotely to this machine.” Administrator should be enabled by default but feel free to add whatever users you would like.
Now go to start, run and type MSTSC.exe (Windows XP) and this will open the remote desktop connection box. Now if the machine is on your network you can type in either its name or local ip address. If it is on the internet you will need to know its external IP address. this can be found by going to www.whatismyip.com from the machine you want to connect to.
After you make the connection it is as if you are sitting right in front of the machine. Amazing!