I am not sure if they are still about but I purchased a copy of a 'student' version of XP Pro which is a full copy, genuine and approved by MS. It has to be verified by MS or you won't get updates and such when you need them.
However, there is a company in Kent called Davad Technology who supplied them (like I said I am not sure if they still do). They can be reached on 01634 710840. They did cost £65.