I'm already using it for newgroups with no problem. The reason it wont become the default news handler is that there is no other news handler installed. Ergo, it IS the default, by default, if you get what I mean.
It was the question of why I could take the default setting away form Incredimail Premium that was puzzling me. But I put it down to the fact that Windows 7 does not support Windows Mail in any case, so it's just probably ignoring the fact that it's active.