How to change os from windows 7 to xp

Set up a Dual Boot Environment. Keep Windows 7, and add XP. XP only has a couple more years of Support.
http://www.sevenforums.com/tutorials/8057-dual-boot-ins...

Buy an XP license, put in disk, follow prompts to install it. Make sure you check your computer to make sure it has XP drivers.

Although if I were you, try Windows 7 a bit longer, not worth the time trying to install XP when you don't have to.