windows xp 64

First: Pc spec; Msi Gt 710 lp, 3gb ram, intel pentium 4 630(Soon to be pentium d 945) Now my main question. How do i downgrade from windows 8.1 to windows xp 64 bit pro? i created a bootable usb drive with rufus (coudn't use win7usb download thing because it said that its not a iso file despite...