Microsoft no longer sells W7, nor was there ever a W7 OEM upgrade edition available for purchase. Some PC's came with a free upgrade to W7 and the upgrade kits came with the PC at the time of purchase.
You may be able to still find a retail W7 upgrade kit or an OEM W7 full installation program on the secondary market such as eBay.
Just make sure that if you decide to buy the OEM software, that you get a new sealed kit.
If an OEM product key was used on another PC, it cannot be used on a different one.
I also believe with a full OEM installation disk, you can't do an inplace upgrade. I do not believe the product key will work for an upgrade, only a full (custom) install.
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