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As of this writing, the only sites I see selling retail editions of Windows 11 are scammy product key websites that I wouldn't trust with my credit card info, so I would recommend buying a Windows 10 license from a reputable site. If you're installing Windows 11 on a new PC you've just built yourself, officially, you should buy a Windows 10 or Windows 11 license. So if you're running Windows 10 Home or Pro on your PC, regardless of whether your PC is officially supported or not, you'll be able to install and activate the equivalent edition of Windows 11. Windows 11 is a free upgrade to Windows 10. Finally, you can just download an ISO file directly from Microsoft’s site. You can also burn the ISO to a DVD, but installing from any USB drive, even an old USB 2.0 drive, will be much faster, so you shouldn’t do that. Once you have a USB drive, you can either boot from it to perform a clean install or run the Setup app from within Windows 10 to do a normal upgrade install.
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The second is to use the Windows 11 Media Creation Tool, which automates the process of creating a bootable USB install drive or downloading an install ISO file. One is to use the Installation Assistant app, which you install on your PC to trigger a normal upgrade install via Windows Update. Microsoft offers several ways to download Windows 11 manually. But because Microsoft is rolling out Windows 11 slowly over the course of many months, your PC may not see it yet. The easiest way to get Windows 11 is by checking Windows Update on a supported, fully up-to-date Windows 10 PC.
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Further Reading Windows 11: The Ars Technica review
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