You can only use 1 cd key per account on steam. If you try to create a new account on steam, typing in a CD key that you have used to create another account before, you won't be able to create it. You need to either login using the account you created using that cd key or get a new one. If you do need to get a new one, you will need to buy Half Life, which isn't really expensive and it will give you access to all HL mods, including opposing force, Day of Defeat and any other 3rd party mods you choose to install.
i ve got 2 cd-keys but i live in Norway and Half-life kost about 300 norwegian money LoL not that much!
