Microsoft .NET Framework version 3.5.1 is included with Windows 7. Follow the steps below to enable it.
If you try to run .NET 3.5 installer downloaded from the Microsoft website on Window 7, you will receive the following error: You must use "Turn Windows features on or off" in the Control Panel to install or configure Microsoft .NET Framework 3.5 SP1. Instead of running the installer, follow instructions on this page.
Note: The same steps work for Windows 8 and Windows 10.
Click Start -> Control Panel
Click Turn Windows features on or off
Click the checkbox next to Microsoft .NET Framework 3.5.1
You will see the checkbox become filled
Wait for Windows to complete the operation. If it asks you to connect to Windows Update to download required files, click Yes.