Tom Coyote carefully read the Windows Vista EULA and noticed that the Windows Vista upgrade editions invalidate your Windows XP serial key.
Here's the snip from the Windows 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.
This means that after you've installed Windows Vista you won't be able to use your Windows XP copy anymore - the activation won't work.