First, I gotta ask, why?
Switching to xp because your "used to it" is like driving a 78 Ford Pinto just because your used to the way that it handles, even though it can blow up.
Now, having said that, you probably shouldn't be doing this yourself because you do need to know how these things work. If you attempt this you will be in a world of hurt and out of a computer for a while. I seriously recommend to find some local help, get a friend that has built computers and have them help you.
Also, another issue that you will run into is that you will need a license for the copy of Windows XP that you are wanting to install.
It's not as straight forward as everyone might think, but do some research and weigh your options before you do anything drastic.