Product key code.

I just bought a Dell Inspiron 530 that has the Operating System Windows XP Professional. It is Microsoft Office Business. I did not know this at the time of purchase. Therefore; I need help in receiving a new product key code so I may use the Word 2007 applications that are programmed into this computer. I am a student and need this information as soon as possible. This computer was purchased from a pawn shop by me on December 03, 2009 and it was purchased by the original business owner on January 14, 2008.