Ok from reading earlier and trying to sort this our for half the day today...I finally got it to work. I wont try to link the posts i found, I will just tell what i did and hope it works for you as well.
Most important thing done, clear cookies for IE and Firefox.
Second, when you log into Crysis 2, the screen where you type in your username and password...
There is a button to modify your key or whatever, click that and type in the serial key, without dashes.
I have not had to retype the code since doin this, and it also eliminated the key in use error i was getting whenever i tried to join multiplayer games.
Hope this helps, goodluck.