It's really a good idea. There is no such thing in Windows as a downgrade path, so you will have to start with a completely new Windows XP installation. However, XP keys are really easy to get from retired computers. And so long as you keep it off the Internet XP is the finest environment for those older games.
In spite of the fact that you can't downgrade from Vista keeping all your programs, settings, favorites, etc intact, I still think that is your best course.
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