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The upgrade invalidates the XP key. The key won't work even if it is on a dual-boot or on another system.
Stated from the Vista EULA:
13. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligible
for the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software
you upgraded from. After you upgrade, you may no longer use the software you upgraded from.