You can reinstall Windows on the same computer as many times as you want because activation pairs the Windows product key with information about your computer’s hardware. If you make a significant hardware change, you might have to activate Windows again.
Question:So when I reinstall, how does my clean install know about previous ones so it will not ask me to re-activate? And what exactly is a "significant hardware change"?
You make a significant hardware change to your computer, such as upgrading the hard disk and memory at the same time. If a major hardware change requires activating Windows again, you will be notified and will have three days to activate your copy of Windows.
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You reformat your hard disk. Reformatting erases your activation status. In this case, you’ll have 30 days to activate Windows again.
Question: How does a clean install of Vista know if it's installed on a new HDD (which if I don't upgrade my RAM or make any other changes shouldn't lead to a reactivation according to the above statement or, in case I upgrade more than just the HDD, give meonly 3 days to reactivate) or on a reformatted harddrive (in which case I will always need to reactivate, but will have 30 days to do so)?
Perhaps I missed something? How could I upgrade the HDD other than by buying a new one and reinstalling the OS?