OEM keys are hardware locked, if you have a desktop and you replace one of the major components in it (CPU, Motherboard etc) your Windows registration will fail and you won't be able to use this same. You'll have to buy another key.
Retail Keys are best as they are not hardware bound, even if you change major components or the whole PC you can still use the same key everywhere. But only on 1 PC at a time.
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