It's the licence key that you need - with that you can borrow a genuine XP(?) master disk and install entering your key when prompted. Did your retailer not place a sticker bearing the key on the underside, or supply you with a manual with the sticker attached? Would your supplier possibly hold this information on record as the computer is presumably still under warranty?
Without that key you do not have a licensed OS as the key is your licence. You may need to buy a new copy.