Windows can be activated any number of times, but your re-activation experience will vary based on the way you acquired Windows.
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If you acquired Windows via retail purchase (boxed product), you may activate via the Internet the first five times. Subsequent activations are allowed but must be completed via telephone.
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If you acquire Windows pre-installed on a computer, re-installation would not require additional activation steps unless significant hardware changes were made.
To activate Windows on this computer, do the following:
Open Windows Activation by clicking the Start button > click Computer > click Properties > click "Click here to activate Windows now". If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
For more information on Activating Windows, please click the following link:
Activating Windows: frequently asked questions
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