To Change your BF2 CD key:

Go to Run.
Type regedit and hit OK button.
Click on plus sign next to HKEY_LOCAL_MACHINE.
Click on plus sign next to SOFTWARE.
Double click on Electronic Arts folder.
Double click on EA Games folder.
Double click on Battlefield 2 folder.
Double click on ergc folder.
Data displayed will probably have x9392 only.
Right click on it and select Modify.
Type in x9392 and then your CD key.

Well this was the advice I was given by EA and I'm sure that it works in certain cases but it didn't work with my whole palaver. In the end I had to get rid of everything and reinstall. I'm back on the game now yay!