Hi Ralph. Not that simple. If you purchased a new computer with XP, there may be drivers involved that don't support XP (especially with laptops). My suggestion would be to contact the maker of the computer and see if they support the "downgrade". In some cases, it may void the warranty.
You will also need an XP installation disc.
If you decide to try it, make sure to back up everything BEFORE you start.
My recommendation is: don't do it unless the computer manufacturer supports it.