XP PRO was considered to be more robust, networked easier, had a few small but expensive things included that was not in the XP Home version.
having used both i only had problems with the networking of XP Home, If you are intending using it as a sole singular computer and not a networked system Home will suit your purposes just fine, as well as you can get the less expensive update versions for Vista, if you decide to go along that route eventually,
XP Pro cannot be updated into Vista (according to microsoft) therefore you would have to purchase a more expensive Vista Full version
