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      Hello... here is Microsofts directions for activation... if a motherboard fails, and is replaced, or upgraded CPU/motherboard, and/or operating system software is re-installed... these are typical reasons for a re-activation. Pretty painless online activation and even over the phone... you have 30 days to activate, or you will get a Black screen desktop. you can even try out Windows 8 or any other of their products for 30 days.
 
http://windows.microsoft.com/en-US/windows7/activate-windows-7-on-this-computer 
 

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mike_mccue
I figured out the black screen thing this afternoon. :-)

Oops.

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There is a batch file utility called the IR5 Trilogy (ver 4.2 seems latest) that enables you to re arm the activation process a few times so you don't have to activate Windows immediately. Good option if you need more than 30 days to test out your hardware. Works for Win 7.