Gamespot has posted a Q&A session with Allen Nieman, technical product manager for Windows XP

GS: You mention that the changes have to do with the amount of time since the key was last used, presumably for either the initial activation or one due to hardware changes. How long are we talking about?

AN: The amount of time is based on initial activation. If you make changes to your PC--changes that would normally prompt a need to reactiviate--120 days from the time you initially activate the product, it will automatically do it online for you.

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