What is a computer activator?

It is a loader. This is the loader application that's used by millions of people worldwide, well known for passing Microsoft's WAT (Windows Activation Technologies). Examples of loaders are: Windows.Loader.v2.1.3-DAZ and 7 Loader eXtreme Edition v3.010. The Spanish technologists call them activadores which may be where the English computer word 'activator' comes from.