Usually when you install a new game or software, you are likely to forget or lost the original CD key that you get with the CD packing after the first install. Or sometimes you might throw the CD cases by mistake which contains the key.
If you have lost/forgotten the serial of your favourite games and now cant find them, I have found a very interesting application that will do the job of finding the keys for you.
Game Key Revealer is a freeware tool that can find out CD keys and serials of installed games on your computer. It displays keys of over 500 games that can be saved as a text or Microsof Word document. You can also print them to keep a hard copy record of them. But make sure you put them in a safe place! 
Game Key Revealer also supports 64-bit edition games.
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