Recently I’ve installed Windows 8.1 Pro 64 bit. I’ve updated the system using Windows Update, and then tried to install applications that I often use, but I couldn’t install those which require .NET 3.5.
I’ve used the OS’s popup thing to download/install .NET 3.5 and always got an error saying that I’m not connected to Internet (although I was).
I also tried to install it from installation media using DISM tool (command is: Dism /online /enable-feature /featurename:NetFx3 /All /Source:<drive>:sourcessxs /LimitAccess), but I also got an error.
Then I found someone who had the same problem, and he noticed that his computer recently installed some updates related to .NET 3.5. Those are KB2966826 and KB2966828. The solution is to first remove those updates…
So, if you have trouble installing .NET 3.5 on your Windows 8.1, try removing installed updates KB2966826 and KB2966828, and then try to install .NET 3.5.
You can search for and remove specific updates in Control Panel > System and Security > Windows Update > Installed Updates.
I found this solution here: http://superuser.com/questions/799687/net-3-5-installation-comes-up-with-error-0x800f0906-then-0x800f0081f-using-dis