You need to verify a few things first.
1) Make sure your system is compatible with XP. Some newer computers dont have drivers for XP only windows 7. So you may need to check you manufacturers site to see if XP is listed under the drivers for your computer.
2) You will need a copy of windows XP, borrow from a friend because you will not find a copy in stores.
3) You need a valid product key to install, sometimes you can use a key off of an older computer, or I believe there may still be some services out there that sells unused CD keys (This will run you ~100$)
But why would you do this? Windows 7 on the proper hardware is a superior system to XP.