Okay, here is what you do.
Download STEAM from
http://www.STEAMPOWERED.com
Install it.
Run it and make a new account.
Log In.
Go to view game list (NOT PLAY GAMES)
At the bottom should be a button that says something like "I have already purchased one of these games" CLICK THAT
You should be asked to enter a CD-Key (Enter you HL1 CD Key)
Then, double click HALF LIFE in your PLAY GAMES list, it will begin downloading.