Why when i try entering a server in Battlefield it says something about the CD Key ?

Sometimes when i try entering a server, It enters and everything is cool.. Other times when i try to enter one, it says something about the CD Key.. What's wrong with it then ??



P.S.: I downloaded the game, didn't buy it.. And i'm using Vista and live in Egypt if that helps.Why when i try entering a server in Battlefield it says something about the CD Key ?
There could be a few reasons, but without the exact error message I can only guess what the problem might be.

If your cd key was invalid you would get kicked everytime, it's possible you have been banned from some servers, or maybe someone has a copy of your cd key and when they are online you can't play and vice versa.

You could try registering your game and cd key to your account, this should stop anyone else using it, but it will mean the game will be locked to your account and you wont be able to sell it when you are bored with it.

You can register the game by clicking on the register option in the start menu, or if it is not there you can find it in the support folder, default location Program Files\EA Games\Battlefield 2\Support EReg