Ed Bott:

For nearly a year, Microsoft has refused to release technical details of the changes it made to its Product Activation technology in Windows Vista. The company was more than willing to speak in broad terms about the program and how it works, but it kept the details confidential, classifying them as trade secrets.

Until last week, that is. A newly released Technical Market Bulletin entitled Product Activation for Windows Vista and Windows Server 2008 unexpectedly appeared on Microsoft’s Download Center last week. Curiously, the document was dated September 2007, but the Date Published field indicates that it was kicking around internally for more than a month before being officially released.

A complete description of the actual activation policy is found at the bottom of Microsoft’s Windows Vista Activation FAQ:

    How many times can I activate Windows Vista?

Windows can be activated any number of times, but your re-activation experience will vary based on the way you acquired Windows.

If you acquired Windows Vista 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.

If you acquire Windows Vista pre-installed on a computer, re-installation would not require additional activation steps unless significant hardware changes were made.

And even that description, [Microsoft] explains, is potentially misleading. The policy allowing five automatic activations over the Internet has been in place for the past year, but it’s subject to change at any time. The real goal, it turns out, is to block hackers who try to spoof parts of the hardware ID so that multiple systems can appear identical when they check in with Microsoft’s activation servers.

Fascinating reading.