Alternatively, locate your original Windows installation USB stick. Mount the ISO or insert the USB into your computer. Installation Methods: Offline
The most reliable "offline" method is using your Windows installation media, as documented by Microsoft. .net 3.5 sp1 offline installer
The .NET Framework 3.5 SP1 is a cumulative update that includes .NET Framework 2.0, 3.0, and 3.5, along with numerous improvements, such as , AJAX support , and performance enhancements to WPF (Windows Presentation Foundation) . along with numerous improvements
Microsoft provides the full standalone package via its download center: .net 3.5 sp1 offline installer
The is essential for air-gapped machines, build servers, and anyone tired of waiting on Windows Update. Keep a copy of the full dotnetfx35.exe or the sxs folder from an official Windows ISO in your IT toolkit.