When I checked Microsoft .NET Framework 3.5.1 under Windows features and clicked OK to install it, I got this error message:
An error has occurred. Not all of the features were successfully changed.
How do I fix this?
When you turned the feature on did you just check the root node and leave the other two sub nodes for WCF un-checked?
The WCF HTTP Activation is used with IIS and IIS should automatically be turned on when you enable this feature but if there was an issue turning IIS on then the error you are seeing may be related to IIS and not .net 3.5.1.