Urgh, newbies...
Anyway, you've installed the game to a certain directory, right ? In that directory there will be a long named executable, something like "Harry Potter And The Chamber Of Secrets.exe". This is NOT the one you have to overwrite, it's the launcher where you can change settings and run the game. If I recall correctly the nocd is called "game.exe", right ? Well, in the same directory where you find the "Harry Potter And..." exec, you have several subdirectories. One of them is called system, and THERE you have to put the game.exe, overwriting the old one. So in your case it should be in "C:\program files\ea games\harry potter chamber of secrets\System\". If you can't find it, tell me what OS you use.