Once you have purchased any original software it becomes your responsibility to keep your product key safe which you may need in future. But it’s likely that you lose it because you had written on some paper or saved it in text file which don’t remember now.
If you still have original software CD with you then you can get back your product key. Here is small freeware utility ProduKey 1.35 which can be useful in such situation. ProduKey lets you to find CD-Keys (Product Key) of MS-Office Windows and other Microsoft installed applications.
ProduKey displays Product ID, CD-key, running application version. ProduKey also supports command line options.