Try:
Type type = Type.GetType(inputString); //target type
object o = Activator.CreateInstance(type); // an instance of target type
YourType your = (YourType)o;
Jon Skeet is right as usually :)
Update: You can specify assembly containing target type in various ways, as Jon mentioned, or:
YourType your = (YourType)Activator.CreateInstance("AssemblyName", "NameSpace.MyClass");