If the drive crashed from a 'soft error' like corruption of the kernel or something, you could try using a Knoppix or other LiveCD bootup to extract the key from the (hopefully still useable) registry on the hard drive. If you can get the WinXP CD 'Repair Console' to boot, you may be able to use that.
Otherwise you are better off finding the CD case with your key, buying a copy of XP, or using free Linux.
Hint: If your hard drive quit mechanically, try tapping it against the heel of your hand and remounting or positioning it inverted (or at least in a different position from what it was). Reboot, and if the drive works and you can boot, extract the key with Belarc Advisor and recover your data before wiping and tossing the dying drive.
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