Just because it doesn't have an XP sticker or a sticker with the key on the case, doesn't mean its not a legit copy. If its built at a Mom & Pop shop, or by a family member or friend, they may have simply gone out and bought it off the shelf.

Personally, I rarely put the stickers that come with CPUs or software on computers that I work on - I'd rather put their logos on my toolbox of geektasticness.