If you want to own the game legally, look for a sealed or used copy of Half-Life Platinum Collection or the original Counter-Strike Retail Box on eBay. Not only do you get a valid key (which can be activated on Steam for modern CS), but you also get a cool piece of gaming history for your shelf.
In the pre-Steam era, "CD keys" were used to validate retail copies. If you are trying to install or activate this legacy version today: counter strike 14 cd key
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