Steve you dont need a CD/DVD for any games you have registered to your steam account, If its in your account you can download it from steam over and over again.
If you goto the games list within steam, If you see "In account" marked on any you can download So say your HL2 CD/DVD got scratched beyond repair, You could log into steam and install it from there.
Hope thats understandable...lol You know what Im like trying to explain things...