As of January 2020, Windows 7 has reached its End of Life (EOL). While you can still install .NET 3.5.1, running Windows 7 on an internet-connected machine without extended security updates is risky. We recommend upgrading to Windows 10 or 11 where possible. However, we understand that legacy hardware and software often require Windows 7.

Yes. The “.1” denotes the version that ships with Windows 7. It is identical to .NET 3.5 SP1 with minor OS integration fixes.

If the "Turn Windows features" method fails, ensure your Windows Update service is running, as it may need to download supplemental files. Is this for a specific app or game that isn't running?

Microsoft deprecated the older TLS 1.0 security protocol on their download servers. Ensure your Windows 7 has the KB3140245 update to enable TLS 1.2. Alternatively, use the "Windows Features" method which does not require internet downloading.

If you are using a different version of Windows (like XP or Vista) or require a specific redistributable for administrative deployment, you can find them on the official Microsoft Download Center Full Offline Package Best for computers without an active internet connection. Web Installer (Bootstrapper)