Hi again, i'm really confused between Product Id and Product Key.
It is good when you're Product Id changed when you reinstall the OS?but still the same Product Key? Is Product ID really important when it comes in Legal Issue.

I think this was summed up at http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/difference-between-windows-7-product-key-and/9de1d99c-f5e4-41e2-9c7b-6b81e39b5b97?auth=1

Product Keys and Product IDs are two completely different things,

despite the similarity of the terms.
The 20 character Product ID is created during the installation
process and is prominently displayed towards the bottom (in the
Activation section) on the opening panel window resulting from
right-clicking the Computer icon and selecting "Properties" from the
context menu. It is used to obtain/qualify for technical support
(limited though that may be) from Microsoft; it is of no use during the
installation process.
The 25 character Product Key is used to "prove" ownership of a
legitimate license and is required to perform the installation, and is
either stored on the DVD packaging on a sticker that says "Do not lose
this number," or is on a Certificate of Authenticity (CoA) label affixed
to an OEM computer.

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