Problems with installing .Net framework 3.5 SP1

I was told to do this:

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"Go to: start/control panel/programs and features

On the top left you should see "Turn windows features on or off" click on that

Find "Microsoft .NET framework 3.5.1" when the window opens

Click the + sign next to it to expand it so you can now see:

"Windows Communication Foundation HTTP Activation"
"Windows Communication Foundation HTTP non-Activation"

Make sure there is a tick in both boxes and select OK."

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But here's the thing:

"Windows Communication Foundation HTTP Activation"
"Windows Communication Foundation HTTP non-Activation"

These are by default unchecked for me. When I check them, I get the following message:

"An error has occurred. Not all of the features were successfully changed."

When I click OK, I says it needs to restart to apply the changes. After restarting and going back to the same places, the above two boxes are still unchecked.

Any help would be greatly appreciated, thank you.