The Windows product key is on the outside of the packaging on the back. Did you immediately rip open the shrink wrap and toss it? That is where your product key SHOULD be... right below the CD on the back of the package. If you still have that shrink wrap, then get it out of the trash... because that's where your key is located.
Timely reminder. Just got an OEM copy of XP and remembering this post, put the sticker with the key on it on the booklet the CD sleeve is attached to. Also always write the Key on the book as well.