I think you are looking at Product ID which is not the same as your cd key.
If you don't have the cd key that came with it google for rockxp. Rockxp is a free program that will give you cd keys your system is using for windows and office.
FYI symantec's av software considers this hacking software and will try to block it.
Some instuctions useing another program(I have not tried these)
http://pcsupport.about.com/od/productkeysactivation/ht/findof2007key.htm