Did you do your homework about this downgrade? You will have to completely wipe the hard drive & start from scratch. That means you will have to locate all the necessary XP compatible drivers for your hardware - chipset, video, sound, network, wireless, etc. Not only that, but your system most likely has a SATA or SSD hard drive, which means it's not natively supported by XP. So there will be extra steps that will have to be taken to get XP to install. If you're familiar with computers & installing Windows, drivers, & all the other related software, it shouldn't be too difficult. But if you're a newb, you're going to have a lot of work ahead of you.
My advice is to stick to Win7 & learn it. XP is the past, Win7 is the future.