First you need to enter the BIOS. This normally means pressing a key, or combination of keys, immediately after switch-on. A message to this effect usually flashes up on the screen when you switch on, such as "To enter setup press Del". Other common BIOS codes are listed at click here.
When the BIOS screen appears you will see a set of menu options. Look for one labelled Boot or Startup and select the item Boot Order, or First Boot; this will be set to HD0, HD1 or the serial number of your hard disk drive.
Change the setting to read CD, CD-Rom or the serial number of your CD/DVD-Rom drive, Save the Change and Exit the BIOS. Open the CD-Rom drive, insert your XP disk and restart your computer.