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go to start > run; type in regedit and click ok.
locate the following key: hkey_local_machine|software|electronic arts|ea games|battlefield 2|ergc
on the right pane there should be something called (default). double-click on this (or right-click and then select modify) to open the edit string window.
type in x9392 and then your battlefield 2 cd key (don't include any spaces or hyphens). i.e. x9392ab12cd34ef56gh78, where ab12cd34ef56gh78 is your cd key.
exit the regedit program and run the game.