Back up all files and folders you want to keep. Downgrading to XP involves formatting your hard disk and erasing all content.

Download the appropriate drivers for your system. The system drivers control your hardware components and if your system came with Vista, they all speak Vista's language. You need to download the XP drivers for your system components from the PC manufacturer's webpage (Dell, HP, Sony, etc). Put them all on a CD or flash drive and save until after the XP installation.

Insert the Windows XP Setup Disk and restart the computer. Press "F8" at startup for boot options and select "Boot from CD."

Follow the on-screen instructions to format the hard drive and install Windows XP.

Activate XP with your product key or call Microsoft and tell them you are downgrading from Vista and they will provide you an activation code.

Install the Windows XP drivers you downloaded before and configure XP to your preferences.