That happens when you're trying to boot up from a hdd that doesn't have windows, or from a cd that is not bootable AND a hdd that doesn't have windows.
EDIT: Assuming you configured the boot-up options correctly and have the Windows XP cd in the correct drive, be sure to press
any key when it displays
Press any key to boot up from the cd-rom... (any key being any key on your keyboard duh
).
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