Basically i bought game through steam 2 days ago, never tried to play online but i did put serial number in, so i thought its assigned to my account. Yesterday i upgraded my PC to Quad core which resuleted in formatiing hard drive. But since i have partitions only did it for C, luckly all my steam files were on partition D, so i just installed steam again, used to same Dir, and all my games has beeen autmatically added up and installed without downloading. Today i wanted to play my Crysis 2 online, but when i try to join the game it says m serial its in use?? I also get notification from STeam saying that this game requiers CD-key and under it i have my CD-Key. And thats the key i use. For some reason it says my serial number its in use? I used the same account for My Crysis, b4 and after format.

Problem fixed by itself, now i can play online and it doesnt come up with serial error, wierd...

I'm having the same problem, personally I'm kind of upset that I pay good money for a game and I can't play with one of my friends who also has the game because my serial key / code is in use?!? that's bull dude, I'm very upset atm.