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#21
Drive letters can be different in boot mode than what they are in Windows, so XP might boot up as C if you install it to another lettered partition from boot. Normally there aren't letters there if you are truly installing from boot.
Did you try writing the XP ISO installer using the tutorial I linked for you?
Is it possible you could install to a separate HD with the WIn7 HD unplugged as this is the easiest way to Dual Boot. After install plug back in the Win7 HD, set it first to boot in BIOS setup. Trigger XP HD when needed using the one-time BIOS BOot Menu key.