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And spreading rumors like that is exactly the ignorant behavior this article was written to counter.

Installing an upgrade copy of Vista makes your previous OS license legally invalid. Why? Because the upgrade license for Vista is contingent on a prior version. This is why the upgrade is cheaper.

It does NOT make the XP key technologically invalid. You can still uninstall Vista and reinstall XP, using your previous key, without any problem.

What you can't do, legally, is upgrade a PC to Vista, and then use your XP key to install XP on a different machine. But the key would still work to do it.