Not sure that's the case with XP - as PA said I think all you get is a recovery disc. Think you have little option but to buy the software. There are various articles online about why that is - anti-piracy being one, monopolistic battle against Linux another.
Cheapest Win XP Home OEM I've seen in UK is £63.92 inc VAT. Search for it using 'Microsoft Windows XP Home Edition OEM Full Version' as search string or PM me for link if required. I bought it no questions asked, on it's own, without buying hardware at the same time.