Half life 2 key problems

[GoodyP]

My dad gave me a physical copy of Half Life 2 to play. It had already been installed on multiple devices before this. I put the disk in my computer and installed the game. It then asked me to register the key but the button sent me to this page.
http://www.vugames-europe.com/Eregistrations/init.aspx?lang=UK
I tried to click on the desktop shortcut but it just sent me to the steam page to buy HalfLife 2.
Have I done something wrong, or is their nothing I can do?

[J4MESOX4D]

Keys can only be redeemed once on a single Steam account. If you want to play Half LIfe 2; you will need to play on the Steam account the key was redeemed or have on/have it family shared to your current account or you must buy the game again. PC games that require Steam even if physical are a one-time activation only and once the key is used; the disk is worthless and useless as is the code.

[GoodyP]

thanks for the help