When you buy an installation disk for XP or other versions of Windows it is for personal use unless buying a corporate license to install it on several machines. With Vista coming out soon you can find some bargains pricewise if you shop around. Don't bother buying an upgrade disk however. You can find low prices on the full version at this point.
There are tons of places that sell XP OEM Home. I paid $85 in July, and it's undoubtedly lower still, now. Check places at Pricewatch, for instance.
Tom