Need Help Changing CD Key

So, I have a legit key I can use.

Everytime I change it to that key using Darky Key Changer, and run the game (not through steam) it gives me a "bad serial number given in setup" message.

If I change my key to a non legit CD key it runs perfectly fine.

Whats the deal?

You might've gotten your key stolen, have you ran any random exes lately?

Nah, the key I got is from a friend who couldn't run the game at all because of his terrible PC.
It's literally fresh off steam.