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Although proprietary computer manufacturers pre-install an OEM copy of Windows on their computers there is nothing to stop an individual installing an OEM version either.
Many people do it because the software is cheaper than a retail version, but the drawback is you cannot transfer an OEM version to another computer as it is tied to the first computer you install it on.
If you see the Genuine Windows logo and Windows is activated you can safely assume the software is genuine.
The product ID is a way of telling Microsoft what support is available to you when you call them. Generally speaking, they will not offer support for an OEM copy of Windows so if you see a product ID with the letters OEM you'll know that support should be sought from the computer manufacturer.
If you buy and install an OEM copy of Windows then you are the one responsible for any support.