Just because you bought a computer that has WinXP preinstalled, that does not mean that the computer builder didn't use an illegal copy. It's done quite often. Sellers can save quite a bit of money by not including a genuine copy with every computer they sell. Even the install CDs are often copied and given to the consumer (printed like a legitimate copy to make it look real).
If the computer is from a well known maker like Dell or HP, I'd be on the phone with them within seconds demanding that they resolve the issue. If the seller is an independent store, I'd first contact MS to verify if the copy is legit or not, and if not, I'd cancel the credit card that I used to buy it, then contact the seller demanding an authentic copy, and threaten to turn them into Microsoft if they don't comply.