Your product key for Office 97, XP, 2003 & 2007 is stored in the registry, so if you still can access the hard drive you can use Magical Jelly Bean to retrieve that key. Magical Jelly Bean (version 2.05 and later) is even able to retrieve keys from a non-bootable hard drive (see item 4 on the link). I have used this approach myself with Windows XP and various versions of Office.
Magical Jelly Bean KeyFinder:
http://www.magicaljellybean.com/keyfinder.shtml This product will not work on Office 2000.