Once you're in XP, you're already booted. You can't boot from a CD once you're in the OS. You boot BEFORE you're in the OS.

The better question is: "how to boot from a bootable CD?"

The answer is:
1. Boot into the BIOS-setup by pressing the magic key (or magic key-combination) very early during a boot. There's no standard for what this key is. Quite often it's shown on the screen ("press del to enter setup" or a comparable instruction). If it isn't it should be mentioned in the motherboard manual or the user manual of the PC. If not, try F1, F2, F10, escape. Or post the details of your PC here.

2. In the BIOS-setup find the screen where you can change the boot-sequence. Change it so booting from CD is the first option.

3. Save the changes.

If the drive is OK, and you put a bootable CD in the drive, it will boot from that now.