up until yesterday i was playing Source with no problems at all and loving it, today i tried to connect and i get a message saying this cd key is already in use. I dont know what to do but i really want to play.. just post under this or message me in on my AIM: Exus013. Thnx.

Has anyone else used your game CD to install the game in their computer (not legal)? Do you own another PC with the game installed, have a broadband connection with a router (which is ok) but forgot to exit Steam?

No, no and no :(

If no one had a chance to see your CD key, and you're the only person who knows it, then there's no chance that someone else is using it - try to remeber if anyone had the chance to see the key and copy it...

Well, there is a chance that someone actually coded a good keygen that generated your key, but guys at Valve swore that the chance to generate an original CD key is astronomically small, but even at 1 to a billion, there's still a chance - I hope that hasn't happened here.

You should try and contact Valve support and see what they say.

I presume you have already have a Steam account with your CD key registered. If so and if someone else came along afterward, you should be fine and they should get a message that the cd key is already in use, not the other way around.

Perhaps it was just a glitch. Try it again. If the same thing happens, you probably should contact Valve/Steam.

Mailpup is right - if you have proof of purchase just take a few screenies of the CD key use rejection and take it to Valve/Steam.

....if you have proof of purchase :)