In this follow up to Replace the Retiring Windows XP with Linux we're going learn the easiest and most foolproof way to install and run Xubuntu Linux 12.04 LTS on your old Windows XP computer. The only requirements are that your old computer has USB ports, and that it supports booting from a USB stick. If your computer is too old for USB, then it's pretty darned old and you really need to think about getting a newer one.
There are but four steps: one, go to OSDisc.com and order Xubuntu Linux on a 32-gigabyte USB stick. Then insert the USB stick into your computer, boot up, and start playing with Linux. It is self-contained and doesn't change anything on your computer. You can create and save files on the USB stick, install and remove Linux software, and it's completely portable so you can plug it in and use it anywhere. If you ever get bored with Xubuntu you can delete it and use your USB stick for something else.
Booting to a USB Stick
Ideally your computer is already configured to boot from removable media like USB sticks, CD/DVDs, and even floppy disks if it's old enough. Boot devices are listed in order in your computer's BIOS (basic input/output system), for example CD/DVD, USB, and then hard drive. Your computer BIOS is a little program installed in a microchip on your motherboard, and it performs the initial startup before handing off to your operating system. The BIOS goes down this list of boot devices until it finds one that is present, and then uses that.