I have a few friends who only got to play Battlefield 2142 before they got an invalid CD Key message. I have no clue how this could happen. The funny thing is, I used both of their accounts on my machine, and it worked perfect for me.
Try to install your game on another computer, or wipe out your system.
I really don't know what else to say. But if all else fails, you can buy a new 2142: Deluxe Edition (with Northern Strike) CD Key at http://www.noobkeys.com for $16 and make a new account and start over on your game. Plus you wont need the CD to be in.
You can simply enter the code on EA-Downloader and have the full game+expansion pack.
I can bet you anything that if I had your account, I could login and play using your data with no problem. If you have a spare hard drive, or create a new system partition, you could install windows again and I bet that it will work. (or again, just wipe out your comp)