No, HL2 and the rest of the gamse on steam are stored on a server, and if you buy an original HL game and enter the key, you will always be able to get CS and so on. Its all registered in your user name. So if you have entered a valid CD key in you user name for HL you will be able to get CS and all the other games anytime cause you have already entered the CD key, it downloads it for you. So i think it will be the same for HL2.
So i guess the answer to your questionis, yes. If you bought it through steam, with your current steam user name, you can install steam on another computer, and enter your username, which tells steam that you have already bought these games, so it will download em for you.