You need the XP operating system CD and certificate of authenticity.
I advise you to stay away from Ebay on this. Too many scams for Operating Systems there.
If you are going to buy the XP system, get it on Amazon.com because if it is a rip-off, in any way, even an opened box or un-sealed CD, Amazon has their "A to Z" claim system that is very supportive for the buyer and they will get you your money back and chastise the seller, if not kick them out permanently.