I know this question is resolved but, for the benefit of anyone else reading it; if you have all of the types involved as strings, you could do this as a one liner:
IYourInterface o = (Activator.CreateInstance(Type.GetType("Namespace.TaskA`1[OtherNamespace.TypeParam]") as IYourInterface);
Whenever I've done this kind of thing, I've had an interface which I wanted subsequent code to utilise, so I've casted the created instance to an interface.