Purchased computer model Elite 9070a

Question

purchased computer model Elite 9070a have been well serviced with HP products this being 3rd HP computer we have purchased. However am very disalusioned with Vista program and wish to replace with windows xp. am unable to install as computer tells us program older than that installed! do not wish to continue with Vista please advise how to remove and replace with xp? If unable to assist will consider exchanging system for Apple/Mac dualThank you Tere Burdett