If you want to get XP on a new computer, you'll need to purchase a copy of Windows XP from a store that still carries it. With Windows Vista and Windows 7 now making XP obsolete, it may be very difficult to find. Future Shop no longer carries Windows XP.
Even when you do have a copy of Windows XP, there's no telling if the newer hardware your computer has will be supported in the older operating system. While the computer might be able to run XP, certain features, such as sound, wireless networking, or proper video may not be available, as drivers may not be written for the older operating system.
If you absolutely need Windows XP to run certain programs, I recommend upgrading to Windows 7 Professional instead -- this version of Windows 7 includes a feature called 'Windows XP Mode,' which allows you to run Windows XP as its own program inside Windows 7. You can read more about Windows XP Mode here.
Hope that helps!
