Activation is set by a number generated by a mix of hardware id's taken from your system. If those numbers change significantly (how significantly? Who knows?) then activation is tripped and you have 30 days to figure it out. I just changed motherboards and processors, and it wouldn't let me activate, but there's a number you call, and you read off this long number to them, and they read back another long number to you which you type in a dialog box, a pretty simple process really, and they say OK and you are activated. With a change in hd it is quite possible you won't even have to do anything, but worst case you have to read a lot of numbers.
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