HL2 / Steam re-installed. CD Key Error.

[ratzzz]

I've had to re-install Steam / HL2, and when entering the key from the HL2 case I get the above error.

How do I get round this if I already have an account registered with my key?

[Link]

Just log in to your account. It will carry on as normal after that.

Your CD key is locked to your account. Aside from proving you own it if you had your steam acccount stolen, you could throw the key away, it will never work again.