Keep in mind a legal copy of XP Home Edition will set you back $189.00. When XP first came out you had to contact Microsoft to get a key. I'm not sure if they still do that. It's not easy to install the same copy of XP on two machines and not have Microsoft find out about it.
You can get an OEM copy of XP Home for about $90. That's what I use when I build Windows systems. There are cheaper ways intended for high volume users, but they start getting a bit sketchy for home users.