Consider using it in an "Air-Gapped" (offline) environment if possible.
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To guarantee that the ISO you downloaded has not been altered or injected with malicious code, you must verify its SHA-1 hash against known official Microsoft hash values. Official Microsoft Windows 7 64-Bit Hashes: windows 7 64-bit iso file download
Microsoft also had a software recovery page that served a similar purpose. It often required a product key and worked for some users, but its availability has been inconsistent since support ended. It's always worth trying the main download page first.
If Microsoft rejects your key, it may be an "OEM" key (came pre-installed on a Dell, HP, Lenovo, etc.). In that case, you need Method 2. Consider using it in an "Air-Gapped" (offline) environment
Although Microsoft doesn't directly offer Windows 7 downloads anymore, if you have a valid product key, you might be able to reinstall it through the Microsoft Store or other official channels if available.
Simply downloading the ISO file isn't enough; you must "burn" or mount it onto a portable drive so your computer can read it before the operating system loads. Official Microsoft Windows 7 64-Bit Hashes: Microsoft also
Windows 7 remains a preferred choice for running certain older software applications or for use with legacy hardware that may not be supported by newer operating systems.
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If your computer has 4GB of RAM or more, you use the 64-bit version.