With STEAM you create a username and password; whenever you login to the system it shows you a list of games that you have bought and then gives you the option to install them to the computer your currently on so you can play them.
I just recently bought a new PC and when I put STEAM in to play Football Manager 09 it came up with a list of all the other games I had previous likw Half-Life 2 etc.
So it should just be a case of putting your username and password in on another computer and waiting for it to install the game for you.