Can't be done without the software being already installed. (If it could Microsoft would have all sorts of trouble with pirated copies, wouldn't they.)
The product key is the most valuable part of the XP installation kit. If you have lost it it's just the same as if you lose your wallet; keep these things somewhere safe in future.
Just as no-one will give you your money back if you lose your wallet, no-one is going to give you an XP key for free. Time to buy another copy.