If Office XP is currently installed and accessible you need only use
the following FREE program to retrieve your product key:
Magical Jelly Bean KeyFinder
http://www.magicaljellybean.com/keyfinder.shtml
If all you have is the CD and don't have the jewel case with the
Certificate of Authenticity you'll have to call Microsoft at
800-936-5700, read to them the CD-ROM part number on the inside rim of
the CD, and describe the artwork on the CD itself, then they will
charge you $10 for a new key. Actually, I don't know if this is the
case for newer software like Office XP. I know this is the way you get
a product key for older software, such as Office 2000. But the 800
number is definitely the right one. I just called it this morning. The
Magical Jelly Bean KeyFinder works well too, if that's possible for
you.
Good luck!