shortbus has it right. If you don't see the CD key there, you no longer have your CD key.
The only way you will have your Cd key is if you have previously entered it into the computer. The key is NOT stored on the disc at all. The game uses an algorithim (math equation) to determine if the key you entered is valid....which allows for an infinate amount of keys to be used, and for them to not have to code each disc with a specific key.
In other words, if the CD key is not in your registry, you will never get it back. HL is cheap.....you chould just grab another copy of it and use the CD key that comes with that.
And just FYI....if you install steam, and it asks for your CD key, then that means you don't have it any longer. When I installed steam, I already had HL installed so it just auto-detected my CD key, because it knows exactly where to look for it. If you don't have HL or CS installed, Steam will ask you to enter a CD key....